RPM @ 80mph with 4.10 vs 3.45? - ZR-1 Net Registry Forums

WVZR-1 · 07-11-2009

George and I have both used the "advertised" diameter for a 315 tire but when I did mine a few years ago I did it with "rolling tire radius" from my car which directly affects the speedometer and it would be interesting to see George's Exel done with that. It's not a major discrepancy but that's how the factory calibration is theoretically done. The number 25.4 seems familiar and that would be 12.7 X 2!

I don't recall what mine was but I did it with a "tram & bubble level". I'll remeasure when time permits and post back. You can also use a single rotation of the tire on the asphalt. Scribe the asphalt & tire at 6:00 and move the car a single rotation of the tire to 6:00 again. That would be "circumference" and that divided by pi (3.1416) would result in the diameter required for a more accurate result.

I've always felt the advertised diameter should do but it does get debated.

07-11-2009	#12
WVZR-1 Join Date: Oct 2004 Posts: 1,890	Re: RPM @ 80mph with 4.10 vs 3.45? George and I have both used the "advertised" diameter for a 315 tire but when I did mine a few years ago I did it with "rolling tire radius" from my car which directly affects the speedometer and it would be interesting to see George's Exel done with that. It's not a major discrepancy but that's how the factory calibration is theoretically done. The number 25.4 seems familiar and that would be 12.7 X 2! I don't recall what mine was but I did it with a "tram & bubble level". I'll remeasure when time permits and post back. You can also use a single rotation of the tire on the asphalt. Scribe the asphalt & tire at 6:00 and move the car a single rotation of the tire to 6:00 again. That would be "circumference" and that divided by pi (3.1416) would result in the diameter required for a more accurate result. I've always felt the advertised diameter should do but it does get debated.