Sterling Doc

Since the debut of the R7, there have been many threads on how the A/R7's are larger than the '6's, but this does not seem to be equally applied to all sizes, and Hoosiers tire specs have become a little suspect as well.

From what I can gather the 225/245's got new molds, and are wider, with the 245's essentially matching the 275's now (which are still the R6 molds), at least in the 17" sizes.

Has anyone compared the 18" 245's to 275's?

I ask because the breakpoints in NASA ST for tire assessments are 245, and 275. If the 245's really are similar to the 275's, it doesn't pay to take the HP (or price) penalty.

I tried the BFG R1S's which are enormous in the 275/35/18 size (barely fit), but I have never got them to work as well with my car/driving style.

Sterling Doc

So I went in the garage and measured the mounted, used tires I have in terms of actual scrubbed tread/contact patch:

245/40/17 R7 on a 9" wheel - 10.25" square shoulder
255/35/18 R7 on a 10" wheel - 10.5" round shoulder
275/40/17 Continental wet on a 9" wheel - 9.75" round shoulder
275/35/18 BFG R1 on a 10" wheel - 10.8" - intermediate shoulder

The 255 was historically Hoosier's "cheater" size, and by their own measurements was the same width, just less radius, than their own 275. Now the 245 is almost as big in terms of tread width (especially with giving up an inch in rim width).

I guess I should run the 245, and be happy, but what I really want is a wide 275, like the BFG.

analogmike

FWIW the 275 / 15s are definitely wider, I smoked one to death on my springplate nut in the 1st session I went out.

Sterling Doc

So to answer my own question, Phil's Tire had the 275's and 245's contact patch measured for me. Less than 1/4" difference in actual tread - the 245's have a square shoulder, and the 275's still have the round shoulder (like the 255's I have).