So Tire Rack recently posted up this test, some of you all may have already seen it:

https://www.tirerack.com/tires/tests...y.jsp?ttid=296

Overall I think it isn't really saying too much new information really, it's stuff we already knew. Pinching wider tires onto a narrow rim has rapidly diminishing returns and mildly stretching a tire can have performance benefits. 255 on a 17x10 has been en vogue for a while now for our cars. What I found interesting was that the 245 on a 17x9 was much slower compared to the 225 than I thought it would be. I know you can't extrapolate test results out too much from their test environments but it's a small detail to piqued my interest. If this is so then why don't STR S2000s and ND2 Miatas run 225 instead of 245 or 255? To be frank, 225 on a 17x9 is a bit more stretch than I would take myself. Where is the knowledge gap here? To me the apparent explanation is suspension setup and alignment. I imagine STR cars with much more camber and roll stiffness can negate the downsides of a more pinched tire setup than a stock BRZ which is camber limited and suffers from much more roll.

What are your all's thoughts? I would like to hear from the hive mind and those more experienced than me

 

I have been scientifically proven as a genius for running 225/45/17 RT660 on 17x9.5.

Proper tire stretch is best tire fit forever.

 

Scientific? Lots of mentions on "feel." More PII. Four different cars for sure. Four different drivers? Unknown "control" tire -- I couldn't find what t was.

And what's the "For Track Use Only. Not to be Driven on Public Highways." mean?

-- Chuck

 

Not for use on public highways is due to tire:wheel pairings outside of what the manufacturer recommends. Its liability stuff.

 

Gotcha. Lawyer speak so Tire Rack ain't responsible (but will sell the stuff ).

Thanks.

-- Chuck

 

In wheel width limited applications (like autox classes) I have known a few fast folks that have tested with various widths. Most all will still say that stuffing slightly more tire on, even if pinched, is still overall better than running less tire width that is a better fit for the wheel. They all have said that yes, when comparing the same tire width on a slightly stretched vs slightly pinched application, slightly stretched is better. But running less than the max width for your class simply to have a tad more stretch on the 9 in max wheel width (str) may not net any gains or may be slower overall.

Another case of what is best with all options available vs trying to maximize contact patch within a set of class rules. I think ES miata folks have seen that even if slightly pinched, running more tire is still better on the limited wheel width.

 

This test would seem to be consistent with our experiences in some regards. It would agree that a street class BRZ is better off stuffing as much tire as possible onto the oem wheel width despite marginal returns. But what about a ST class BRZ? On a 17x9 wheel it would seem to imply that they would be better off with a 225 than a 245 or 255 (depending on offset). Obviously that's not the actual case out here on the autocross pad. I think there's probably too much noise here to really extrapolate whats going on in that regard. Where do we lay the explanation to this? How much are driver preferences playing in this? I'm curious what peoples' thoughts are on this. Especially in regards to a class where you won't be limited by the oem BRZ suspension setup. Like how STR S2000s run 3X the OEM spring rates and the Karcepts bar.

 

They did say the 245 on the 17x9 set the single fastest lap but the average was lower than the 225 on 17x9 so I think you're right about too much noise to really tell what is going on.

 

Yeah, I noted that too. How much of that is an outlier, and as a tangent, how repeatable would that have been for a driver more comfortable with the setup? To me it sounds like the stretch was being used to compensate for driver and setup preferences. This test has been living rent free in my head for a while. Particularly because I'm considering pinching 275s on my 17x9.5s.

 

I think the test results would not necessarily translate over to a ST type setup where you have alignment options. But i do agree some stretch these days is required as tires are much wider than they used to be.
i've tried a052 245 on a 9, 255 on a 9 and 255 on a 10. Did a back to back 245 vs 255 on 9's and a back to back 255 on 9/255 on 10. Obviously the 255 on a 10 was quickest. and i preffered the 245 over the 255 on a 9.