What Gives???

[richiedd]

Alright, i'm running M/T Sportsman Pro 26/12.5/15 on a 10" rear rim as a street tyre with some grip when needed.

The specs on the tyre says that with that rim the section width should be 11.9". However, when I measure the actual tyre/rim combination I am getting 12.5"!!

I am wanting to order new wheels and tyres but do not know how reliable the posted specs are likely to be, given the above example. What has been the experience out there? Is it just a case of M/T being "off" and I can trust the specs posted generally, or is there a caution that we all should be aware of with these tyre specs?

Thanks,

Rich

[DriverzInc]

I want to make sure I'm understanding you correctly, the tire size is a 26/12.5/15 on a 10" rim, right? And when you measure your actual tire section width you are getting 12.5? Sounds consistent to me.

ON a 15" rim, a tire will bulge pretty far out because of the height of the sidewall, and the weight of the car. Something to keep in mind.

Are you you going to a larger diameter rim? In most cases when doing this, the tire bulge will be at a completely different radius from the center of the rim, and be more "square". And the stated specs on larger diameter wheels and shorter sidewall tires will have less magin for error as they won't "bulge" much differently if mounted on the correct, recommended rim with range.

For example if you are considering 17x10 inch wheels, and you are thinking about 275/ wide tires, the tire manufacturer's section width will be pretty much dead on.

I hope some of that makes sense. If not, shoot me an email, or call me anytime. Maybe I could explain it more clearly.

[richiedd]

Thanks Jon....I understand your explanation perfectly. It's just that M/T gave the 11.9" section width using a 10" wide rim as their recommended wheel for those specs. I'm not sure why when I use their recommended wheel, I am getting a wider section width.

I am planning to change to a 315/35/17 tyres in an 11" wheel. The specs that I am seeing out there indicate a section width of 12.5", more or less. I want to make sure that that will fit in the wheelwell where the M/T's will be vacating (M/T using a 5.5" b/s, new wheel to have a 6.0" b/s). Any thoughts on this approach?

Thanks,

Rich

[DriverzInc]

Give me a call if you can, but your 17x11 and that tire are going to be exactly what the specs say it will be, in regards to section width, but it sounds like that you understand that it will be a wider tire, at a higher radius from wheel center as it will not bulge out and curve inboard as much as your M/T does, which is why you must be going with the 0.5" more backspace.