# Calculating Tire Size

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**31**for example: 285/70/17

285x70=19,950

19,950/1270=15.70

15.70+17= 32.7

you will allways divide by 1270 after you get the product of the 1st 2 numbers, after you get that number you just add the rims size and their you go. 3 simple steps to get your answer.

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**34**Senior Member

Join Date: Dec 2010

Location: boise, id

Posts: 1,130

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**35**Junior Member

Join Date: Oct 2011

Posts: 2

CAn you buy beefy 35" tires that fit your stock rims?

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**38**Member

Join Date: Oct 2012

Location: NE Ohio

Posts: 69

Jeremy

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**40**
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**44**Technical Article Contributor

Join Date: Mar 2012

Posts: 154

Since everyone usually has questions on tire and wheel combos/sizes. I decided to do some calculations and help members out with metric to U.S. standard(inches). This information is all over the net, however, I thought it would be convenient for it to be all in one place. I only used the most relevant and commonly used truck tire sizes. Therefore, this could be beneficial and educational to the general member with no tire experience.

I see alot of posts implying that a 305 70 is a 33 x 12.5 or close to 33. When in reality, its impossible to calculate the height of a metric tire without the rim size. The diameter of a metric tire must be calculated using all three of the tire specs. I think this confusion is because most American tires do not depend on numerical calculations; the specs posted are the respective height and width regardless of depending on rim size. For instance; a 33 x 12.5 x 15 has the same height and width(within reason) as a 33 x12.5 x 17. However, a 305 70 17 is not the same height as a 305 70 15.

Alot of people get confused when talking about metric tire sizes. Generally, this is because the numbers reflected in the metric sizes aren't exactly width and height as we typically like to think.

The first number represented (the largest) is the sectional width. This number in MM is how wide the tire is from inner sidewall to outer sidewall. This does not include protruding ***** or ribs. Also note, these calculations vary by rim width, but only by a few tenths of an inch. So for all the calculations and specs I will assume we are using a factory standard size wheel.(the oem equiped wheels).

The second number represented (usually the 2nd largest) is the aspect ratio. This spec. is exactly what it says; a ratio. To be precise it's a ratio representing the percentage of height to the section width. When doing calculations this number will always be a decimal because its a ratio/percentage. For example; A 305 70 17 has a section width of 305mm and an aspect ratio of 70 percent(or .7).

The third number represented is the rim size in diameter(usually the smallest because this number is in inches). This spec. is the diameter or height of the rim you will be mounting the tire on. No calculations to be done for this one.

Now that we understand what all the numbers represent we can start to analyze and convert our specs to U.S. Standard. However, before we begin, we need to know the conversion of mm's(millimeters) to inches. I won't mathematically calculate this cause its easy to find "online". The conversion is 1" = 25.4 mm.

I will once again use the 305 70 17 as our example. So we have ;

Sectional width = 305mm

Aspect ratio = 70% or equivalently .70

Rim diameter = 17"

To get our sectional width to inches do;

305/25.4 = ~12.007 inches

This means from inner sidewall to outer sidewall(not including *****) our tire is 12.007 inches.

Below is a picture representation. Note: on mud tires your true width will be greater than this. (Typically because of the *****). Also note: In the picture I am showing the top of the tire; The exact representation should be taken from the widest part of the sidewalls, inner to outer.

Now we multiple our sect. width by our aspect ratio. Numerically we have 12.007 X .7 = 8.405. This calculations is to see how tall(diameter) "one part" of the tire is. By "one part" I mean the top half from one part of the rim to the outer edge of the tire(obviously, there are two of these and they will be the same size because its a circle). Please remember, all tire representations are on mounted rims, but not loaded on a vehicle(the pressure and down-force of the weight changes these measurements).

See picture;

Now we can calculate our height by adding the top half+ bottom half + rim diameter to give us the total tire diameter. Hence, why we cannot assume a 305 70 15 is the same as 305 70 17, because the calculations are dependent on rim size.

Picture representation of all numerical values;

Using our example(305 70 17) we have;

8.405" + 8.405" + 17" = 33.81"

Thus a 305 70 17 is approx. 33.81 inches tall

Comparatively, a (305 70 15) is;

8.405" + 8.405" + 15"= 31.81"

It is easy to see the discrepancy between the sizes.

Again the calculations go;

305/25.4 =12.007 (sect. width in inches)

12.007 * .7 = 8.405" (top and bottom halves)

8.405" + 8.405" + 17" = 33.81" (complete diameter)

I understand this is a very rudimentary and fairly abstract way to understand metric tire conversions. However, I think it will be fairly useful to prospective tire buyers. Please note: THESE ARE APPROXIMATIONS. All tire specs and guidelines differ. A Goodyear 305 70 17 may have a few incremental differences between a Nitto 305 70 17.

To help members out(so they don't have to do the calculations) I did the metric conversions for some of the most used tires and rim combos. See the reply(too many characters for me to post on this thread). These are not guaranteed to be exact nor do I take responsibility for indiscriminate errors. Always research your tires for fitment and other general questions.

*** If you feel I have made a mistake please let me know, these calculations were done simultaneously and fairly quick. Thanks in advance.

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**45**Technical Article Contributor

Join Date: Mar 2012

Posts: 154

For 17" rim and aspect ratio 65;

[265, 0.65, 17]

10.43307087

30.56299213

[270, 0.65, 17]

10.62992126

30.81889764

[275, 0.65, 17]

10.82677165

31.07480315

[280, 0.65, 17]

11.02362205

31.33070866

[285, 0.65, 17]

11.22047244

31.58661417

[290, 0.65, 17]

11.41732283

31.84251968

[295, 0.65, 17]

11.61417323

32.09842520

[300, 0.65, 17]

11.81102362

32.35433071

[305, 0.65, 17]

12.00787402

32.61023622

[310, 0.65, 17]

12.20472441

32.86614173

[315, 0.65, 17]

12.40157480

33.12204724

For 17" rim and aspect ratio 70;

[265, 0.70, 17]

10.43307087

31.60629921

[270, 0.70, 17]

10.62992126

31.88188976

[275, 0.70, 17]

10.82677165

32.15748032

[280, 0.70, 17]

11.02362205

32.43307087

[285, 0.70, 17]

11.22047244

32.70866142

[290, 0.70, 17]

11.41732283

32.98425197

[295, 0.70, 17]

11.61417323

33.25984252

[300, 0.70, 17]

11.81102362

33.53543307

[305, 0.70, 17]

12.00787402

33.81102362

[310, 0.70, 17]

12.20472441

34.08661417

[315, 0.70, 17]

12.40157480

34.36220473

For 17" rim and aspect ratio 75;

[265, 0.75, 17]

10.43307087

32.64960630

[270, 0.75, 17]

10.62992126

32.94488189

[275, 0.75, 17]

10.82677165

33.24015748

[280, 0.75, 17]

11.02362205

33.53543307

[285, 0.75, 17]

11.22047244

33.83070866

[290, 0.75, 17]

11.41732283

34.12598425

[295, 0.75, 17]

11.61417323

34.42125985

[300, 0.75, 17]

11.81102362

34.71653544

[305, 0.75, 17]

12.00787402

35.01181103

[310, 0.75, 17]

12.20472441

35.30708662

[315, 0.75, 17]

12.40157480

35.60236221