Why the max 35 aspect ratio on 315s

[Live Wire]

Maybe a silly question... I'm not sure if this is always the case for every manufacturer, but for Toyo, Nitto, and M/T it's the case. It's always 315/35R17 (or less) whereas the 275 are usually 40R17. Is there some technical reason that 35% of a 315mm tire is the max height? I'm guessing there are physics at play that tire manufacturers don't challenge, but curious for a little more insight.

Thanks,
LW

 

[Rich'sTRsvt]

Those two sizes you mention are nearly the exact same side wall and that is the reason for the commonness of that size. Tire companies make tires in sizes that car companies offer cars with oem. So more than likely there is a car that requires a 315 35 17 but not any with a higher side wall in that width and size.

 

[Black02GT]

Keeps the tire diameter the same. The profile is a percentage of width (not a straight measurement like 35mm,40mm, etc) so as width goes up aspect goes down to keep the same diameter. M&H makes a 325x45x17 but that's probably a 28" tire vs 315x35x17 which I think is 26.5ish. This will change your effective final drive. 315x35x17 is similar diameter as 245x35x17 stock.

 

[Live Wire]

Right, and I didnt even take the math into consideration even though I knew 35 means 35% of 315

When you do the math, 35% of 315mm is 110.25mm. 40% of 275 is 110mm. Nearly identical sidewalls like you guys are saying. Makes total sense now.