Can 4.80/4.00-8 Tires Be Upgraded To Larger Size For Higher Speed Rating

Question:

I have a boat trailer for my 200 lb wood rowboat. The existing tires are 4.80 x 8 and rated B. The only driving I do is turnpike, usually 60 - 70 mph. Now I see about the speed ratings so what would be the best tire and wheel to buy? I have new 4 on 4 CE Smith hubs coming from you tomorrow.

asked by: Mark J

Expert Reply:

Hello again Mark; as long as you don't exceed the 65 mph rating you can stick with a tire like the Kenda Loadstar K371 Bias Trailer Tire # AM10002. Moving to a larger tire with a higher speed rating is going to mean a fairly large size difference. To see if you can use one of these tires you will need to measure for clearance.

The closest thing that also uses an 8" wheel would be the Kenda 215/60-8 # AM3H290. This has a speed rating of M for 81 mph. However, it is significantly wider with a section width of 8.9" and also taller with an outer diameter of 18.3". Compare this to the # AM10002 and you will lose 1.1" on top and 2.15" on the inside of the tire. You need at least 3" on top and 2" on the inside for proper clearance.

The other option would be to move up to the Taskmaster 4.80R12 Radial Trailer Tire # TA49QR which also has an M speed rating. The width won't be much of an issue since it is 4.84" but the outer diameter is even greater at 20.63". Comparing it to the same tire, you would only lose 0.12" on the inside which should be fine but the top will lose 2.265".

If you have fenders on there you could remove them to gain the clearance on top. Either way you will need to compare these to the clearance with your current tires to see if either one would work.

expert reply by: Robin H

Mark J.

10/5/2024

Going with a wider tire sounds nice (it might make for a cushier ride for the old wooden boat) but it’s getting close to the frame, and taller may hinder ease of launching, so I’ll have to stick with the current size. That said the tire/wheel set AM30040 looks like a good match, and has a C rating. Perhaps I’m getting confused about “load” ratings vs “speed” ratings. But I’m all about looking for a soft ride as the lapstrake boat is fragile. Would a B actually be softer?

Robin H.

10/5/2024

@MarkJ I understand. We get asked about changing tire sizes a lot and most of the time it just isn't possible because of the design of the trailer. Load range ratings like B and C indicate the ply rating of the tire which is also associated with the tire capacity. A load range B tire like # AM10002 above is 4 ply and a load range C like the # AM30040 is 6 ply. The higher ply rating does make it a little stiffer so I would stick with the load range B. For a wheel and tire combo you can use # AM30000. For trailer tires, if they don't have a speed rating listed on the tire you can assume they are rated for the standard 65 mph which is technically a rating of K. Higher ratings on trailer tires are going to be L for 75 mph, M for 81 mph, and N for 87 mph.