RPM @ 80mph with 4.10 vs 3.45?

[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

[WVZR-1]

I suspect you're tach is in "error"!!


You mention 6th gear which is .50 and I'd guess that 2500 RPM would calculate with a 315 tire (25.68) to maybe these speeds:

3.45 = 110 -112 MPH

4.10 = 93 - 95 MPH

4.77 = 80 -82 MPH


You could confirm the tach error with a scanner connected to the ALDL.

I've always tried tach/mph in a gear that was 1:1 (4th) gear in our case. It seems substantially easier to maintain a steady MPH. You could use a GPS to confirm the "speedo" first and then just drive the "theoretical" MPH and check the Tach!

Drive 55 MPH and the tach should be 2500 +/- --- 3.45

Drive 56 MPH and the tach should be 3000 =/- --- 4.10

I believe the key is to start with a "value" you can confirm, in this case the speedo with a GPS and then determine the accuracy of the tach. My car when purchased using the tach in the cluster had me assuming that it might have already had "a gear" but the SOTP feel just wasn't there. I confirmed my tach error with a Tech1 from the ALDL!

[jonszr1]

my car with 410s at 2000 is 72mph

[PhillipsLT5]

aprox 2500 at 75mph, Haibeck correct speedo gear, 4.10 rear

[Polo-1]

about 2 mpg's less :mrgreen:

[Aurora40]

FYI, you can compute it. At any speed in any gear, the rpm's will be 1.188 times as high as it was stock.

@80mph, stock is around 1,850rpm. With 4.10's that would be 2,200.

[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!

If a speedometer is in error it's "propotional" and consistant. My tach error certainly wasn't consistant. It could be 10% through maybe 3500 and 14% or more above that RPM range. It was certainly a challenge when I first bought the car and considered changing the rear gear. Until I confirmed the RPM's with the ALDL I surely was quite confused.

I did measured miles several times and then finally checked it with GPS. The confirmation with the Tech1 and Diacom came a little later.

I posted the 55 & 56 MPH numbers because it's very easy to drive by the "speedo" at those speeds and reasonably calculate the tach error! I used the tach values at "hundreds" to make it maybe easier to see the error on the tach face.

BTW-I didn't get my tach calibrated this winter and mine idles @ nearly 6500 now and is buried at 15 MPH most of the time. From time to time it falls to a reasonable close value! Not often.

[Gunny]

If you want an Excel gear ratio calculator, click here (http://www.zrwon.com/gear_ratio.xls)

George Jones

[WB9MCW]

WAY KOOL CHART GEORGE

http://i126.photobucket.com/albums/p110/HAMMERHATE/ZR-1345GEARSPEEDVSRPMCHART.jpg

[WB9MCW]

I FIGURE THIS TO BE THE RUN

http://i126.photobucket.com/albums/p110/HAMMERHATE/ZR-1345GEARSPEEDVSRPMCHART-RED.jpg

[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]

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. :cheers:

[Gunny]

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]

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??

http://img.photobucket.com/albums/v346/zrwon/183mph2.gif