Tire sizing from mm-in

[JeepinDoug]

I'm shopping for a set of tires and trying to figure the measurements in a language I understand. Let me start with the factory tires on my truck.

To be practical I'm using numbers only;

265/70/17

265 is the width of the tread in millimeters.

70 is the percentage of the tread that measures the sidewall per side.

17 is the rim diameter in inches.

To convert into inches we'll start with the 265mm

1 millimeter is approx . 0394 of an inch.

. 0394 x 265 = 10. 441

This number gives us the width of the tread, approx 10-1/2" wide.

The sidewall, per side, would be 70% of the tread width.

10. 441 x . 7 = 7. 3087

7. 3087" is the measurement of one sidewall.

To find the actual tire diameter we add two sidewalls and the rim diameter;

7. 3087 + 7. 3087 + 17 = 31. 6174"

So my stock tire, (265/70/17) measures 31. 6174" tall.



I'm shopping for a tire that measures around 33" tall using the same rim. If my calculations are correct a tire with the metric notation 295/70/17 will put my tire at 33. 2722" tall.

These calculations are never perfect with the other variables thrown in, tire pressure, rim width and weight of vehicle but it's a good ballpark figure.

Do my calcs seem close or am I'm missing something?



I'm also looking at the NittoGrappler LT295/70R17D1 121R B. Anyone know the load rating on this tire? Is it D or B?

I'd love to get the Bridgestone Revos but I can't wait for a taller tire, plus I think the load rating is really low, having a hard time finding the correct info.

 

[crymepayz]

Those calcs seem pretty close I have always used this formula take the 265/70/17. There are 25. 4 mm in an inch so take the width of 265 and multiplyby . 7 then again by 2 then divide by 25. 4 then add your rim hight of 17 and that should also be close



265 x. 7=185. 5

185. 5 x 2=371

371/25. 4=14. 6

14. 6 + 17=31. 6

 

[rbattelle]

I ran similar calculations over in this thread. Your calculations seem consistent with mine.



-Ryan

 

[rbattelle]

So my stock tire, (265/70/17) measures 31. 6174" tall.

That is one PRECISION tire!!



[Strickly a good-natured joke! No offense intended!]

 

[KCJackson]

One correction of your definitions... the 265 is not the width of the tread. It is the section width, meaning from sidewall to sidewall at the widest point (which includes some bulge width over the actual tread). Your math is correct, though keep in mind that the calculated height is for an unloaded tire, and tread type and depth can also add to the overall height and revs per mile.

 

[JeepinDoug]

That is one PRECISION tire!!



[Strickly a good-natured joke! No offense intended!]

Yea, I know, I'm am machinist who works in . 0001" increments all day long so the four place decimal point is typical.

 

[JeepinDoug]

One correction of your definitions... the 265 is not the width of the tread. It is the section width, meaning from sidewall to sidewall at the widest point (which includes some bulge width over the actual tread). Your math is correct, though keep in mind that the calculated height is for an unloaded tire, and tread type and depth can also add to the overall height and revs per mile.

Point taken, thanks. I knew there was a possibility of the total width being described in the number.