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How to Determine Tire/Wheel Diameter

How to Determine Tire/Wheel Diameter

If you want to know if a particular spare tire cover will fit, or are trying to find out how much clearance you would have when switching to another tire/wheel combo, there is an easy way to find out. Simply read the numbers off the sidewall of your current vehicle or trailer tire:The most common tire sizes will have three parts, just like the 205/75-R15 tire pictured below. This number will tell you three things about the tire and wheel combination that you are trying to fit: the tire width in millimeters, the sidewall aspect ratio, and the diameter of the wheel.
Tire Size Identification Numbers
Tire Width Measurement
The First NumberThe number 205 represents the section width of the tire, in millimeters. This is the measurement of the tire at its thickest point. If you want to convert centimeters to inches, divide the number of centimeters by 2.54. 1 Inch is about equal to 2.54 centimeters. Example: 20.5 cm / 2.54 = 8.070 in A 205/75-R15 tire would have a section width of 8.070 inches.
Tire Aspect Ratio Measurement
The Second NumberThe number 75 refers to the sidewall aspect ratio. This will help you find out the height of the sidewall, from rim to tread. This is also called section height. The second number in a tire size indicates the ratio between tire width and height.Example: 8.070 in x 0.75 = 6.053 in A 205/75-R15 tire would have a section height that is 75 percent of 8.070 inches.
Wheel Diameter Measurement
The Third NumberThe last number, 15 in our example, represents the diameter, in inches, of the wheel on which the tire is mounted.Note that you do not measure the whole diameter of a wheel to get the diameter (weird, right?). Wheels have what's called an "actual diameter" and an "overall diameter." The actual diameter is what wheels are named after—a 15" wheel will have an actual diameter of 15" (measured between its bead sets). The wheel's overall diameter (measured from the outside edges of the wheel) will be closer to 16".
Overall Wheel Diameter vs Actual Wheel Diameter

Determining Total Diameter

Now that you know the section width, section height and rim size of the 205/75-R15 tire and wheel, you are able to determine the total diameter. The section height, or sidewall on the tire, is 6.053 inches. Multiply this by 2 to determine the amount of tire extending out on both sides of the wheel. Imagine drawing a straight line across the tire and wheel, directly across the center.Example: 6.053 in x 2 = 12.106 in Add the combined section heights of the tire (12.106 in) to the wheel size to get the combined diameter of the tire and wheel.15 in + 12.106 in = 27.106 inSection Height Section Height = Section Width x Aspect Ratio Tire and Wheel Diameters Total Diameter = (Section Height x 2) + Wheel Diameter
Total tire measurement: 27.106 inches

Exceptions

Not every trailer tire will have a size like the example above. We also offer sizes such as 4.80-12 that do not automatically include all the information you need to determine the total tire and wheel diameter. In the case of a size like 4.80-12, 4.80 is the section width of the tire in inches. The final number indicates the wheel diameter, similar to the more common tire size notation. If the first number of the size is 3 or 4 digits separated by a decimal point, you need to look at the last digit to determine the aspect ratio.For numbers that end in 0, the aspect ratio is 92 percent. Numbers that do not end in 0 have an aspect ratio of 82 percent.Example: 4.80 in x 0.92 = 4.416 in A 4.80-12 tire has a section width of 4.8 inches, a section height of 4.416 inches and a wheel diameter of 12 inches. The combined diameter of the tire and wheel assembly is 20.832 inches.
Total tire measurement: 20.832 inches
Written by: Raymond P.Updated on: 6/2/21

Questions and Comments about this Article

Brett D.

gvwr for boat is 2800lbs and trailer is 1000lbs But still cannot find what the center hole diameter is or how to measure it. looks like all 6 bolt patterns are the same , 4,5,8 bolt patterns can be different. or that is what the video says. just looking for a spare rim and tire.

Etrailer Expert
Reply from Les D.

@Brett D. So the boat and trailer have a combined 3800 pounds GVWR. Great, what bolt pattern did you find? What is the pilot hole size? Are your wheels attached with flat head bolts or tapered lug nuts? Bolts are for "bolt centric" attachment and we will have to know the pilot hole size. If you have tapered lug nuts, then you are "lug centric" and we will know the bolt pattern.

Reply from Brett D.

@LesD

Etrailer Expert
Reply from Les D.

@BrettD do you have lug nuts or bolts? How many per wheel?

Brett D.

Looking for a rim and tire for 2013 Four Winns boat trailer 190 gs boat. 6 lug and tire size is 225/75d15 , replaces h78-15st Wants to know center hole opening.?? Not sure what he means

Etrailer Expert
Reply from Les D.

The "center hole opening" is the pilot hole in the middle of the wheel. I have attached an article about that. You should also measure the "bolt pattern" using the other attached article I have linked below. A new tire and wheel must be rated to support your trailer and boat weight. What is your trailer's GVWR?

Reply from Brett D.

@LesD boat is 2800lbs and trailer is 1000lbs

Henner E.

I want to lower the ground clearance of my camping trailer so it stands more or less horizontal when coupled to my car. The tyres fitted now on the trailer are Goodyear Wrangler AT/SA 215 R15 109/107 T (no aspect ratio given) and result in a fairly steep downward angle of the trailer when coupled to the car. Because of a factory-fitted tow bar that can fold away underneath the rear bumper It is not possible to raise the ball or tow bar. Will fitting 14" wheels and suitable tyres lower the ground clearance by about half an inch and fitting 13 inch tyres lower the ground clearance by about 1 inch? Please help me, thank you, Henner.

Etrailer Expert
Reply from Les D.

I would like to help with this. What is the year make and model of this vehicle? Are you sure there is not an aspect ration marked on the tire? Could this be a European tire size? We are going to need the aspect to figure out which tire would be lower in height.

Reply from Henner E.

@LesD Hi Les, thank you so much for replying. The camping trailer I have was made here in South Africa (not Australia) by a small company called Offtrax Off Road Trailers which seems to have gone under as they no longer have a website anymore. The model is called Outplay. The trailer's basic statistics are L x W x H 3900x1725X1250mm, GVM 1800kg, Tare 440kg. GA, V and A all 1800kg. I could not find the aspect ratio on the tyre itself but a tyre dealer here said it was 80. The tyre has the words "Made in South Africa" on it. I hope this helps. Regards, Henner.

Etrailer Expert
Reply from Les D.

You are saying that your current tire is a 215/80-R15, which will be 28.5 inches tall from the ground to the top of the tire. You will need a tire with a smaller aspect number for a lower profile. For instance, a 205/75-R15 like the Kenda Loadstar # AM1ST92 is 27.1 inches tall. Watch your load ratings as smaller tires carry smaller loads.

George

Suggest a change: the Dimensions on Your diagrams often show “overall diameter” of the wheel but to be correct the “actual diameter” of the bead is used to calculate sidewall height or tire outer diameter. Maybe easier to understand but not accurate? Am I wrong? But a great article otherwise.

Etrailer Expert
Reply from Jon G.

You are correct. If you check out the "The Third Number" section at the top of the article you will see that we address the overall and actual wheel diameters. The rest of the diagrams are drawn slightly inaccurate as far as the lines go but it's just to help make things look a little more clean and easier to understand. If you were to measure an actual 205/75-R15 tire the sidewalls would be slightly different since they technically start inside the wheel, but the overall diameter (which is the number we are really getting at) will be the same. Hope this helps!

Rich

My tire size reads 235/80R22.5G 134/131M Could u tell me what size motorhome tire covers I should order please. Thank you

Etrailer Expert
Reply from Jon G.

Your tires have about a 37" diameter so that's the range you'll want to look at. Here is a link to our selection for you to filter through.



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