My local tire store has informed me that two of my wheels are warped, and while the vibrations aren't terrible I'd like to try to fix the problem before this year's autox season starts. They wear a set of recently-bought Toyo R888R tires in 225/45R13, and I remembered that I had a pile of spare wheels and tires in my garage that I thought had the same sizes. The tires are old, but perhaps I could put the new tires on the other wheels. But when I pulled them out, they weren't the same size at all.
Obviously, the replacement rim is much wider. The tire appears to be wider too, but looking at the tire markings...
They're the same except the "P". I always thought the two sizes were basically interchangeable, but is that not the case? The old race tires look a little stretched, but that seems like a big difference to make up with stretching alone. What am I missing?
"P" just means that they are tires meant for passenger cars. I'm sure the difference in width is down to variation between different manufacturers.
Keep in mind, tires are measured at their widest point, not at the tread width. And for a given size, they're measured on a specific wheel width. So when stretched onto a wider wheel, they'll measure wider. And tread width can vary significantly between tires of a given size as can the overall shape of the tire.
I always thought that the first number denoted tread width, but that makes much more sense, thanks!
Remember as well that tire makers have rubber rulers
There is an enormous difference from manufacturer to manufacturer in any given size.
Tire dimensions are also nominal, kind of like a 2x4 actually being 1.5" x 3.5". It's called a 2x4 because that was its measurement when they sawed it from the tree before it was planed down.
Tires are also measured on a specific rim width. A 225mm tire on a 7" rim will be narrower overall than if you put it on an 8" rim.
This chart should clear up some stuff.
Notice it gives an acceptable range of rim widths, but specifies on which rim the tire specs are physically measured. Also note that this tire (Conti Extreme Contact BTW) measures 8.9" section width which translates to 226.06mm. They therefore call it a 225mm tire because that's the closest. Now compare it to this Riken Raptor in the same size
Notice that this tire has the same overall width (at the widest point), but the tread width is a half inch larger
In a nutshell, Hoosier cheats on the tire size. This is why some racing orgs have changed the rules so that what's printed on the sidewall is irrelevant and they use a template to measure the tire width.
And here I thought a number was a number haha, this has been very enlightening. I guess this means I'll probably have to replace the warped rims with new ones that will fit the tires I already have.
codrus (Forum Supporter) said:
In a nutshell, Hoosier cheats on the tire size. This is why some racing orgs have changed the rules so that what's printed on the sidewall is irrelevant and they use a template to measure the tire width.
This is the truth of the matter. Hoosiers fit big, like pants from Old Navy.
