Tire Sizes and Revs Per Mile - Duke?

[mike cobine]

While checking the archives to find revs per mile for the standard size tires on Corvettes, I ran across a note by Duke that had me wondering.

Posted 13 Aug 2004
On of the three V-rated 225/70R-15 police pursuit tires. (see the spring and summer, 2003 Corvette Restorer issues.) These have slightly lower revs per mile (760) than the OE F70-15s (775). During the C2 era GM did not change the speedo gearing from the 6.70-15 (760 rev/mile) to the 7.75-15 (775 revs/mile). They just accepted the 2 percent higher readings, so if anything, your speedo will be more accurate with 225/70s rather than 215/70s, which are 775/revs/mile. Also, as the tires wear, the revs per mile increase - about 2 percent from new to the wear bars. Duke


So is the 7.75-15 actually shorter than the 6.70-15? While looking at BFG spec page on Radial T/As, the rev/mile drops as the tire gets taller. I can accept that the F70-15 may have been shorter than the 6.70-15, but the 7.75-15 I would have thought was bigger all the way around, not just width.

What got this started was wondering what in a 16 inch tire would be close, as there are more performance tires available in 16 than 15, and 16 would not look as out of place as the 17 and 18 inch wheels.

For the record, BFG lists:

225/70-15 27.4 dia 759 revs/mile
255/60-15 27.1 dia 768 revs/mile
215/70-15 26.9 dia 773 revs/mile

[Duke W.]

Re: Tire Sizes and Revs Per Mile - Duke?

Yes, the 7.75-15 is slightly less OD and has higher revs per mile than the 6.70-15. All the data is in the referenced Corvette Restorer article, and is based on GM and tire manufactures data I have in my files going back to the fifteis. There can be slight variation between specs from different manufactures. The article discusses tolerances promulgated by The Tire and Rim Association.

When changing tire or wheel sizes you should try to maintain near the OE revs per mile unless you want to change speedo gearing - each increment of driven gear tooth count yields about five percent variation on C2s, but I don't think you will find any 16-inch performance tires near the OE revs per mile. Most that will fit in the wheelwells are much higher.

The Tire Rack and tire manufacturers web sites list dimension data and revs per mile for all sizes.

Duke

[mike cobine]

Thanks, Duke

I'll go compare and see what I can find. I see that 235/60-16 is 27.1 and 768 so it is close. And being a 60 series, it won't look like those rubber bands.

Also 215/65-16 is 27.0 and 770.