RPM @ 80mph with 4.10 vs 3.45?

[WVZR-1]

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.

[Aurora40]

Originally Posted by WVZR-1

Bob,

That works well if the tach is "correct"! It didn't seem to me that the OP's tach was correct and I mentoned confirming the speed which is quite simple with GPS and using a little "a$$ - backwards" logic to confirm the tach error! I assumed that the OP has a 4.10 and that it's got some sort of modification for the speedometer correction. Does he? Don't know!

Sure, but he just asked what the expected rpm was at 80mph with 4.10's. That's what it is. If his isn't that, sure it could be plenty of problems causing it.

[Gunny]

Originally Posted by WVZR-1

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

You're absolutely correct ... using the "advertised" diameter is only "theoretical" but probably doesn't have a major discrepancy ... at least not for "comparison" purposes. And here's yet another curiosity ... the default on the Excel file is set for the Z's standard rear tire, but if you change to the front tire size, the diameter changes slightly. And what about differences in tires by different manufacturers even though the tires are stamped the same size? Yes, the debate goes on.

George

[VetteMed]

Follow -up -- My tach is definitely inaccurate -- using the climate control readout, there's a discrepancy of anywhere from 300 to 500 rpm, from my initial rough calculations. I will get more accurate readings next time out!

[ZRWON]

Originally Posted by VetteMed

Noticed that the rpm at 80mph is around 2500 rpm(as indicated on tach) in 6th gear, which seems high to me. Can anyone please verify, what rpm is expected with stock gearing vs 4.10 or other ratios?
The online gear calculators tell me 4.77 ratio when I enter in the rpm/mph/tire size -- that can't possibly be right?!
Thanks!Andrew

I also have 4.10 rear and really enjoy not needing to down shift from 6th to 4th when doing 70 and want to pass someone.
Following picture is some more speed/RPM data to muddy the waters for you all...BEAST2 has seen 202 in 6th gear that I know of...maybe more that someone else knows of??