7 Questions Everyone Has About Trailer Tires

Typically, whatever experience or knowledge we have concerning tires comes from dealing with our vehicles. This is a great place to start, but it's also where some of the confusion about trailer tires comes in. Are trailer tires the same as passenger vehicle tires? Do they need to be balanced? How long do they last?

Whether you're replacing a blown out tire, upgrading to a larger one, or simply learning as much as you can about trailer tires in preparation, we've got you covered.

In this article:

Do Trailer Tires Need to be Balanced for Proper Performance?
What is Load Range on a Trailer Tire?
Can I Use Trailer Tires on My Car/Truck (Or Vice Versa?)
How Long Do Trailer Tires Last?
Why Do My Trailer Tires Keep Blowing Out?
Why Are Trailer Tires Bias Ply?
How Can You Protect Trailer Tires from Dry Rot?

Watch Now: Our resident trailer expert, Jake, answers the most pressing questions we receive about trailer tires

Do Trailer Tires Need to be Balanced for Proper Performance?

The short and sweet answer: No.

The "DIY guy" answer: You can have your trailer tires balanced if you wish, but this isn't required. When cruising the highway in your passenger vehicle, you're more likely to feel the vibrations from unbalanced vehicle tires, so vehicle tires are dynamically balanced using a machine and weights.

Trailer tires are different in that their main responsibility is supporting a load behind a tow vehicle, not providing a smooth ride for passengers.

Pro Tip: If you buy new tires for your rims, you can choose to have them balanced if you prefer. Remember that many trailer wheels are centered by the position of the lug bolts (these wheels are lug-centric) and not the center bore of the wheel. For best results, have them balanced using a pin plate adapter. This mimics the way a lug-centric wheel is mounted to a hub.

Pictured: a load range E trailer tire

What is Load Range on a Trailer Tire?

The short and sweet answer: Load range basically tells you the load capacity of the trailer tire.

The "DIY guy" answer: The tire's load range is rated with a single letter, such as B, C, D, etc. If you have a load range D and B tire of the same size, the D tire will have a higher weight capacity.

Keep in mind that load range varies between tire sizes. That is, a size 175/80-13, load range B tire might have a 1,100-lb capacity, whereas a size 205/75-14, load range B tire may have a capacity of 1,430 lbs.

For help determining your potential load ranges based on tire size, check out our chart here.

So which tire load range should you use? In most cases, you should select tires with a load capacity that matches the load capacity of the tires that came with the trailer. If you wish to select tires with a higher load capacity, keep in mind that other components of the trailer also have a weight limit, including the wheels. This means that upgrading to higher capacity tires won't necessarily increase your trailer's load capacity—you'll always be limited by the lowest rated component in your setup. Plus, higher capacity tires usually cost more.

Pictured: ST tire (top) and LT tire (bottom)

Can I Use Trailer Tires on My Car/Truck (Or Vice Versa?)

The short and sweet answer: No (with the exception of LT tires).

The "DIY guy" answer: ST (Special Trailer) tires are made for a specific purpose—supporting the vertical load and cornering forces of trailers. P (Passenger) tires are designed with more flexible sidewalls and are made with passenger comfort and steering in mind. Generally speaking, it's a bad idea to swap them. They simply weren't made to perform in the same way.

There are a few exceptions. LT (Light Truck) tires are sometimes used on larger trailers and fifth wheels due to their stiff sidewalls. LT tires are typically readily available locally, and at affordable prices. They also generally have higher speed ratings than ST tires. If you decide to go with LT tires, make sure they're within your load carrying capacity.

Typically, though, you'll want to stick with ST trailer tires for your trailer. That's what they're made for!

This tire was manufactured in the 17th week of 2018

How Long Do Trailer Tires Last?

The short and sweet answer: 3-6 years.

The "DIY guy" answer: In general, you want to replace your tires about every 3-6 years, but it depends on the tire. In most cases, tread wear isn't the problem for trailer tires. Rather, the biggest problem trailer tires face is that they spend most of their time doing nothing. They are typically parked in one spot between tow jobs, soaking up harmful UV rays, gaining flat spots, dry rotting, and otherwise deteriorating due to oxidation.

Even when stored in a controlled garage environment, tires are still subject to dry rot and flat spotting. And even when they look fine from the outside, it's usually a good idea to replace tires once they're about 5-6 years old at most, or you've run them for about 10,000-12,000 miles.

How Old Are My Trailer Tires?

There's an easy way to tell how old your trialer tires are: it's written right on your sidewall. The DOT (Department of Transportation) ID number code is stamped on one side of the tire. The code begins with the letters "DOT" and is followed by numbers or letters identifying the manufacturing location. The last four numbers of the code represent the week and year the tire was built. For example, the number 0418 means the fourth week of 2018.

Tire Overinflation and Underinflation Wear

Overinflation and underinflation produce distinct tire wear patterns

Why Do My Trailer Tires Keep Blowing Out?

The short and sweet answer: You're probably doing something wrong. (Don't worry, it happens.)

The "DIY guy" answer: There are a lot of reasons tires fail. Here are some of the most common:

Excessive heat caused by underinflating
Overloading the tire
Surpassing the speed rating (listed on the tire sidewall)
Flat spots or sidewall rubbing (which could indicate clearance or braking problems)
Bent spindles (less likely but still a possibility)

Aside from checking the psi with a tire gauge or tire pressure monitor system, an easy way to tell if you're underinflating a tire is to look at its wear pattern. If you notice uneven wear on the outer edges of the tire, it's probably underinflated. In the same vein, if you notice uneven wear down the center of the tire, it's most likely overinflated. When properly inflated and adjusted, your tires should wear evenly.

Pictured: bias tire (left) and radial tire (right)

Why Are Trailer Tires Bias Ply?

The short and sweet answer: They're not. At least, not all of them.

The "DIY guy" answer: There are two type of trailer tires: bias and radial. The main difference between them is how they are constructed. The cords inside a bias ply tire run at a 32 degree angle to the direction of travel, whereas cords on a radial tire run at a 90 degree angle, or across the tire from wheel lip to wheel lip.

So what does this mean for tire performance? Radial tires flex more, which provides better ground contact, traction, stability, and tread wear. A radial tire will normally run cooler than a bias ply tire, and since cooler tires last longer, radials generally outlast bias ply tires. At one time, radials were more costly than their bias ply counterparts, but that's changed, and the two types are now similar in price. Radial tires are great for highway use, making them the more popular choice for modern trailers and campers.

So why would you ever buy a bias tire for your trailer? Bias ply tires have stiffer sidewalls, which is great for applications like agricultural use, where you won't be spending most of your time on the highway.

Whichever tire type you choose, just keep in mind that they are not meant to be mixed. Mixing the two types on the same trailer can cause uneven wear and tracking problems.

Covering your tires helps protect them from the elements

How Can You Protect Trailer Tires from Dry Rot?

The short and sweet answer: Work 'em, then wrap em'.

The "DIY guy" answer: There are some measures you can take to keep your trailer tires healthy and strong. Dry rot is caused by excessive heat and sun, lack of use, and low inflation. Use a wheel cover when your trailer is not in use to guard against UV rays, water, and dirt. Jack up your tires or park on plywood boards when possible to keep your tires off the ground when you're not using the trailer. Make sure they are properly inflated (use a tire gauge or tire pressure monitor system to check—trailer tires can look fine and still be underinflated). For long-term storage, remove your tires and store them in a cool, dark area.

How to Measure the Bolt Pattern of a Trailer Wheel

Measuring a Trailer Tire

How to Determine Tire/Wheel Diameter

Written by: Amber S.

Updated on: 2/7/22

Questions and Comments about this Article

David C. 3/12/2022

I own a small Ironton Utility Trailer and recently changed the 2 factory 4.80-12 Bias Ply tires the trailer came with for 2 Taskmaster 4.80R12 Radial Trailer Tire - Load Range C tires. My question is would it be ok to run a spare tire using 1 Bias Ply Trailer Tire 4.80-12 Load Range B as an emergency back up. Say just to get me to a tire shop where I could then get the proper repair and/or tire?

Reply from Les D. 3/15/2022

@DavidC if all the tires on the trailer have the same measurements (height, width, diameter) you could use them at the same time on the trailer. However you would be limited to the lowest speed rating, and the lowest weight rating. Also know that your tires probably have different air pressures and should be inflated accordingly. This by it self could cause them track slightly different. I would only use this spare as mentioned, and at a reduced speed.

Reply from David C. 3/16/2022

@LesD I appreciate the response. I figured I would ask because of the fact that some vehicles come with that tiny spare tire that’s limited to 55mph & a really short distance. My hope was that this concept could be applied to trailer use as well. Yes I would only be using the spare in a limited fashion, making sure I fill the psi to its proper pressure and making sure I haven’t gone over its preferred weight limit. Also I will keep in mind possible tracking issues. In fact now that you’ve mentioned all of this I’m thinking of taking the bias ply tire off completely and installing the same size radial tire. This way I will not have to think about all of do’s and don’ts on our trip and having to find a trailer tire shop along the way. Thanks again for enlightening me!

John G. 2/1/2022

Ordered 14 ply tires for a single axle travel trailer that sees a lot of off road while fully loaded, mostly for durability and toughness but a bit of added weight capacity will be nice also. I already swapped on an overkill axle after bending two of them. The sidewalls indicate something to the effect of "high capacity rims required." Will they work on standard five spoke steelies? Is the "requirement" due to higher PSI and weight rating capability of the tires only, or is there a difference in construction that requires a different style of rim. I will not be running max pressure or coming anywhere close to the tires max weight rating. Thanks in advance, folks.

Reply from Les D. 2/2/2022

@JohnG we would like to help you with this but we need a bit more information. First, what is the GVWR of your trailer? This is the maximum allowed total weight of the fully loaded trailer, and will probably be printed on a sticker on the side of the trailer. Next, what is the weight rating of your upgraded axle. Next, what is the weight rating of your wheels? This will be stamped on the back side of the wheel, usually on a spoke or near bolt hole. Finally, what is the wheel diameter and bolt pattern? I have attached a document that shows how to measure this. All in all, we first want to see that all the equipment is within specifications. Next, tires should be "Special Trailer" ST type as they have stiffer side walls. Bias Ply tires will be better off road as well. Finally, no tires will survive driving off a cliff. There are limitations. If you can reply back with the information requested we can make a recommendation. Also, what size tires are you using now? This will be stamped in the side wall and will look something like ST205/75-14. I look forward to hearing back from you.

Helpful Links

faq-wheel-bolt-pattern.aspx

Jaycee 1/17/2022

Have a utility trailer w/ load tange D 8-14.5’s on 6 lug dexter axle with brakes. Want to switch to a more readily available tire & wheel combo. Thinking st225/75r15 or st205/75r15 any advice one way or the other?

Reply from Les D. 1/18/2022

@Jaycee Out of the tires we sell, we carry the most choices in those two sizes. As commonality goes, either will be available as needed. The 225 is 4.2% (1.2 inch) taller and 9% (0.9 inch) wider. It is not too much difference as long as you have room in your wheel well and between tires. However in many cases these larger tires will let you move from load range D to E. Since both sizes are easily available, I would go with the 225, the more capable tire.

Scott S. 1/10/2022

Where can I get Run Flat Tires for a single axle trailer, 205/75/15 ? I can't believe run Flats are not easily available for Trailers. I've always had a dual axle travel trailer but I'm down sizing to a single axle and I want run flats to avoid a blowout in the mountains or bridges. Can I use LT run flat tires ?

Reply from Les D. 1/11/2022

@ScottS Run-Flat tires are not yet available for travel trailers. Further, I don't know if run flat tires will be created for trailers in the near future. The loads that trailer tires are required to carry, and the stiff tire sidewalls necessary may not be reasonable. For that reason passenger tires and light truck tires are not recommended. Even if they were available (and you know they will be expensive) I would first purchase a TPMS like the TireMinder # TM89FR that would give you advance warning so you don't dammage an expensive tire - be it run-flat or not.

Reply from Scott S. 1/11/2022

@LesD I thought I saw on your website that a LT tire could be used? It's only a 3800lb trailer so I thought a LT tire rated at 2400lbs each side would work.

Reply from Les D. 1/11/2022

Light Truck (LT) versus Special Trailer (ST) tires is an ongoing debate in many places. I guess I will not be surprised to see conflicting opinions here. In my opinion, ST tires are made for a specific purpose—supporting the vertical load and cornering forces of trailers. LT tires are designed with more flexible sidewalls and are made with passenger comfort and steering in mind. I would always use ST's for trailers. Regarding load rating, I would pick a tire with a safety margin in the load capacity (like you suggested) and would also check the speed rating as compared to your driving habits. For instance, the Provider # TR20515D is an ST205/75-15 with 2150 pounds per tire load rating and 81 mph speed rating. (Regarding your 2400 pound LT tire - which tire was that? Are you sure you were not looking at a steel wheel rating rather than tire?) I have attached a link to other valuable tire information.

Helpful Links

dept-pg-trailer_tires_and_wheels-tw-load_range_d-gw-205%7c75~15.aspx

faq-trailer-tire-frequently-asked-questions.aspx

Darrell G. 7/9/2021

I just this week replaced one tire on a twin axel trailer due to a cut sidewall; the remaining three tires were newish and had maybe two thousand miles on them. The replacement is a different brand so I measured the circumference and found it is 3/8” smaller than the others. The trailer hauls a race car and spares, so it’s a little heavy. I’m assuming I need to replace at least the other tire on the same axel?

Reply from Jon G. 7/9/2021

Ideally you want all 4 tires to be the same exact size. You could either purchase another tire for the other side of the axle so that the whole axle is 3/8" smaller or you could buy a new one to replace your new one - just make sure it's the exact same size as your other 3 tires.

Craig 6/15/2021

I have a alluminum boat with a 25 HP motor. In total boat/trailer and motor is about 1000 lbs. my dad sold me the boat and I he put on new tire/wheel bearings prior to the trip. The tires he installed were 4.80 12 (4 ply) with a Bload rating…needless to say at the 7 hour mark of the highway drive I had both new tires flat. I need new tires and from what I can see, I’m reading that I should have had 4.80 12 (6ply) with a C load rating? Is this recommended for highway driving?

Reply from Jon G. 6/22/2021

It just depends on what your boat trailer weight is really. I recommend checking out the Taskmaster 4.80R12 # TT48012C which is rated for 990 lbs (each tire) and has a max speed rating of 81 mph.

Jadatis P. 6/12/2021

In Europe sometimes even normal car tyres are used on light trailers. If maxload of tyre is with comfortable reserve they can sometimes even last upto 10 years. ST is not used in Europe," for trailer use only" sometimes, for max speed 87mph(N) and for that reason 2 loadindex steps higher then same sise and plyrating/ loadrange LT or XL. ST often 6 LI steps more, because calculated in maxload for 65mph, even the Endurance with speedcode N. When I calculate pressure for Traveltrailers, I first derate the FTUO by 2 steps, and ST by 6 steps, to give them highest pressure with maximum reserve, but no screws trembling loose or doors open after the ride.

Reply from Jon G. 6/12/2021

There are quite a few people who use vehicle tires here in the US as well. We try to really encourage trailer tires for regular use but it's not uncommon for someone to build an off-road trailer and slap some Jeep tires on it so it performs better for their application.

Will K. 6/9/2021

I need to replace tires on a 2000 rook wood pop up camper. Old tires are 5.30-12. What would the modern sizing be?

Reply from Jon G. 6/9/2021

It's actually the same size! Attached is a link to our selection of this size tire/wheel combo for you to filter through. You can find something that fits your style and has the same bolt pattern to match your hubs.

Helpful Links

dept-pg-trailer_tires_and_wheels-gw-5.30~12.aspx

Eric O. 6/6/2021

I just put new endurance tires on my trailer. The trailer sticker says 50 psi while the new tires say 65 for max load. I am thinking I need to follow the trailer sticker and stay with 50 psi for the weight of my trailer but don’t want to damage my new investment by under inflating them. I am sure I don’t need the max load of the new tires as this is a lightweight trailer.

Reply from Jon G. 6/9/2021

It kind of depends on the size of your Endurance tire. Attached is a link to an expert question page that addresses your situation. On this page is a chart that shows the weight rating of various Endurance tires when inflated to different psi ratings. In short, you can put less psi in your tires but you need to make sure that you won't be overloading them.

Helpful Links

question-346945.html

Reply from Jadatis P. 6/13/2021

@EricO Because the 65psi = D-load tire is stiffer then the 50psi= C-load, it has less surface on the ground at same deflection, so lesser load to carry at same pressure. For that reason you need a bit higher pressure for the same load. And probably the 50 psi was only yust enaugh with no reserve. In the end this can mean that you better use 65 psi. If you give tirespecifications and total weight of trailer, or better weighed axleload, I will calculate a safe highest pressure with no screws trembling loose and no doors open after the ride. American pressure/loadcapacity lists give all the loadranges of same sise in one list, and pigheadedli I dare to state that is wrong. In Europe for every loadrange/ plyrating a seperate P/LC list is given.

Bill H. 5/12/2021

I have 4.80-12 tires currently on my trailer. What is the largest I can go without changing rims?

Reply from Jon G. 5/13/2021

5.30-12 size tires is the biggest size tire you can go with. Here is a link to our selection. Just be sure that the 5.5" section width and 21.9" outer diameter still allow you to have at least 2" of clearance to your trailer frame and 4" of clearance to your fenders.

Helpful Links

dept-pg-trailer_tires_and_wheels-gw-5.30~12.aspx

Charles J. 4/13/2021

I built my own trailer and put 12” tires on it. Do I want full inflation on my tires when it is unloaded? The trailer itself is less than 390lbs but when fully loaded pops up another 1000lbs. Does using the same tire pressure for both trailer weights cause uneven wear on my tires?

Reply from Jon G. 4/15/2021

This is kind of a controversial topic right now to be honest. Basically since the creation of the world all of the trailer tire manufacturers have said that you ALWAYS run the tires at the psi rating on the sidewall no matter if you're fully loaded or not. Recently though a handful of manufacturers have changed their tone and have released progression charts that let you know what the optimal psi rating is for that specific tire. I would hop over to the website of the manufacturer for the tires you bought and see if they have a chart like this somewhere.

Reply from Charles J. 4/20/2021

@JonG Okay thanks JonG I’ve seen someone somewhere post a weight - missing weight percentage but that seems so roughly done. I’m also wondering should my tire be inflated to max psi before the trailers loaded or checked and made to max psi when fully loaded?

Reply from Jon G. 4/22/2021

@CharlesJ If it were me I would inflate it to max PSI prior to loading up the trailer and then double check and adjust the PSI once you're loaded.

Reply from Ed C. 10/26/2021

@JonG I'm really happy you mentioned this. I've never believed that trailer tires should always be run at the maximum pressure regardless of the weight they're being asked to carry. In my limited experience with trailers, running at the maximum pressure with a light load causes all kinds of bad behavior on the road, like bouncing and wandering, and probably contributes to strange wear. I'm considering upgrading to a load range D tire for my 5'x10' utility trailer, partly because of the increased load capacity and maximum speed ratings, but also because my top choice right now is the Goodyear Endurance, which is only available in that load range in the size I need (ST205/75-15). I've seen the table on their website that shows the load/inflation information for these tires, which is very helpful. Do you know of any other manufacturers who've published similar tables? I've looked at Carlisle, Hankook, Kenda and Power King, and haven't seen anything from them.

Reply from Les D. 10/28/2021

@EdC I would like to address two things. First, tires should be inflated to the maximum cold PSI as stated on the tire in order to obtain that tire's rated weight handling. For instance, if your tires are rated for 800 pounds each at the maximum PSI, but then you inflate to 10 pounds under that rating, then your tire will not be able to handle 800 pounds. However there are a couple of exceptions. Some tire manufacturers will give the best PSI for a known weight, like a travel trailer, but this is rare. For instance, for the very popular Michelin Endurance ST trailer tires, the manufacture publishes a chart what tire pressure to use based on load. Also, sometimes when you have an EMPTY flat bed trailer, users will lower the PSI a bit to keep an unloaded trailer from bouncing around over bumps. In this case it does not matter much as you are not anywhere close to needing the full weight carrying capacity of the tire. Finally, you can measure and adjust your PSI with your trailer loaded or unloaded. Since the volume of the tire does not change when under a load (only the shape) the pressure does not increase. In an extreme situation you may see an increase of around 1 psi.

Jack A. 4/6/2021

I’m looking for tires and wheels for Utility trailer I’m still in the planing stage. I’m thinking of 2 types in one trailer Dump and where the Deck drops to the ground and 12’x6’trailer may be 2 or 3 axle and capacity as hi 10,000 lbs..What would you recommend for tires an wheels axle to fit.. 16” or 17.5 wheels I like Hercules tires What do you think? Any help would be appreciated

Reply from Jon G. 4/7/2021

What you want to do is determine what the weight rating of your axle(s) will be and then make sure your tires are rated to handle at least that much weight. Then you need to make sure that you have at least 4" of clearance from the tread to the fender and 2" from the sidewall to the trailer frame. We don't have the Hercules tires but from what I've looked into I really like the Goodyear Endurance tires. For your application you might be able to use part # 724860519 which is a 235/85R16 sized tire rated for 3,640 lbs at 80 psi.

Richard L. 1/16/2021

Just replaced 12-in tires on my trailer for 13-in tires. I remove the fenders, purchase new ones and welded them on with plenty of clearance, but it seems that my trailer is now harder to pull. Any ideas?

Reply from Jon G. 1/19/2021

The inside sidewalls of your tires aren't rubbing against your trailer frame are they?

Jstrom 12/4/2020

I had a flat on my boat trailer with 5.30-12 tires. I’m seeing a TON of tire with wheel sets for sale but I ‘think’ I’d rather just have the tire replaced. Still looking (online ??) but the last 30 minutes have been somewhat frustrating.

Reply from Jon G. 12/4/2020

Well we do have a handful here on our site so your search is over! I'm attaching a link to our selection that you can filter through. I would check out the Taskmaster # TTWSF53012C specifically.

Helpful Links

dept-pg-trailer_tires_and_wheels-sf-tire_only-gw-5.30~12.aspx

Jeb 9/14/2020

I just bought a utility trailer to haul a raft and accessories. It currently has 8" tires (4.8-8 power kings) and 5 by 4.5" lug spacing. Trailer was used for hauling atv/snowmobile but I would like to upgrade to a bigger tire for the bumpy dirt roads. Can this be accomplished and do you recommend a tire? I have about 3.5" of clearance to decking and plenty of space sides. Thanks

Reply from Jon G. 9/15/2020

You are going to need to raise your trailer or something if you want to use a larger tire, and that's because we recommend having at least 4" of clearance from the top of your tire to the fender (in your case the deck) to accommodate for movement in the suspension. If you were to move to a larger tire then you would likely see contact between your tire treads and the deck of your trailer which obviously is not good.

Ray C. 8/26/2020

Going to FLA pulling my golf. Cart will 13 inch tires do the job

Reply from Jon G. 8/28/2020

If you have 13" wheels on your trailer that should be sufficient to carry a golf cart. You will want to make sure that your tires have a weight rating that match or exceed the weight rating of your trailer axle, and you want to make sure they have at least 4" of clearance to your fenders and 2" of clearance to your trailer frame.

Helpful Links

dept-pg-trailer_tires_and_wheels-pc-13_inch.aspx

Bill K. 8/22/2020

Great information! I'm looking for tires for an older trailer. Ideally, to match, I'm looking for 13 wheel p165 /80 d13 (has 5 lugs). We can't find them anywhere. Any ideas?

Reply from Jon G. 8/28/2020

The closest thing we have is the size 175/80-13. We have this available both in bias and radial, with or without a wheel. Here is a link to our selection.

Helpful Links

dept-pg-trailer_tires_and_wheels-gw-175%7c80~13-pc-13_inch.aspx

John 8/20/2020

I have just purchased a 91 Coleman Roanoke Pop Up camper. I need to replace the tires, currently 4.80z12. I can't even find this tire. If I understand correctly, radial tires are better to interstate travel so what will replace this tire?

Reply from Jon G. 8/28/2020

If you're been searching using 4.80oz12 then that might be why you aren't finding this size. We do have the size you're needing in radial as the Taskmaster # TT48012C .

Brian W. 2/23/2020

Great info. I just purchased my first pop up camper for the family. Its an older camper 1995 Jayco Eagle 12. The previous owner did not know how old the tires are. They look fine, but I want to replace them anyway. They are Super Trail 20.5x8.0-10. Since we will be traveling 2 - 3 hrs by interstate to get to the places we want to go, I assume I am going to want radial tires instead of bias ply. What would the radial version of my tire be? size conversion. Thanks

Reply from Chris R. 2/25/2020

The modern size equivalent to the 20.5x8.0-10 tires on your camper is 205/65-10, however there isn't a radial ply unit available in this size. You'd need to bump up to 12" tires and wheels until you're able to upgrade to radial tires. If you have some room under the fender you could go with the Kenda ST145/R12 # AM10140 or the tire/wheel combo # AM31215 , which would be about 0.6" larger in diameter/height. I do recommend going to radial for better highway driving if this conversion is possible, but if it's not due to fender clearance or bolt pattern, continuing with 205/65-10 bias units like # AM1HP56 would work just fine.

Helpful Links

Tires-and-Wheels/Kenda/AM10140.html

Tires-and-Wheels/Kenda/AM1HP56.html

Tires-and-Wheels/Kenda/AM31215.html