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07-09-2009
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#1 |
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Join Date: Mar 2009
Location: Orlando, FL
Posts: 1,828
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RPM @ 80mph with 4.10 vs 3.45?
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 Last edited by VetteMed; 07-09-2009 at 05:18 PM. |
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07-09-2009
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#2 |
 Join Date: Oct 2004
Posts: 1,889
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Re: RPM @ 80mph with 4.10 vs 3.45?
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! Last edited by WVZR-1; 07-09-2009 at 08:56 PM. |
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07-09-2009
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#3 |
Join Date: Sep 2006
Location: lone pine and mammoth lakes
Posts: 1,407
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Re: RPM @ 80mph with 4.10 vs 3.45?
my car with 410s at 2000 is 72mph
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07-09-2009
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#4 |
Join Date: Aug 2005
Location: Scottsdale, AZ
Posts: 2,450
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Re: RPM @ 80mph with 4.10 vs 3.45?
aprox 2500 at 75mph, Haibeck correct speedo gear, 4.10 rear
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07-10-2009
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#5 |
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Join Date: Sep 2005
Location: Washington
Posts: 955
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Re: RPM @ 80mph with 4.10 vs 3.45?
about 2 mpg's less
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07-10-2009
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#6 |
Join Date: May 2005
Location: Leesburg, VA
Posts: 2,713
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Re: RPM @ 80mph with 4.10 vs 3.45?
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.
__________________
Bob Saveland Former owner of #2517 [IMG]http://a.random-image.net/aurora40/vette.jpg[/img] |
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07-10-2009
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#7 |
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Join Date: Oct 2004
Posts: 1,889
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Re: RPM @ 80mph with 4.10 vs 3.45?
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. |
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07-11-2009
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#8 |
Join Date: May 2005
Location: Georgia Mountains
Posts: 926
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Re: RPM @ 80mph with 4.10 vs 3.45?
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07-11-2009
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#9 |
Join Date: Dec 2003
Location: McHenry, Il.
Posts: 6,607
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Re: RPM @ 80mph with 4.10 vs 3.45?
WAY KOOL CHART GEORGE
__________________
http://i119.photobucket.com/albums/o...9mcw/SIG-2.jpg http://i119.photobucket.com/albums/o128/wb9mcw/vcm.gif "The Budget should be balanced, the Treasury should be refilled, public debt should be reduced, the arrogance of officialdom should be tempered and controlled, and the assistance to foreign lands should be curtailed, lest Rome will become bankrupt. People must again learn to work instead of living on public assistance." - Cicero, 55 BC " We the people are the rightful masters of both Congress and the courts, not to overthrow the Constitution but to overthrow the men who pervert the Constitution." - Abraham Lincoln |
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07-11-2009
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#10 |
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Join Date: Dec 2003
Location: McHenry, Il.
Posts: 6,607
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Re: RPM @ 80mph with 4.10 vs 3.45?
I FIGURE THIS TO BE THE RUN
__________________
http://i119.photobucket.com/albums/o...9mcw/SIG-2.jpg http://i119.photobucket.com/albums/o128/wb9mcw/vcm.gif "The Budget should be balanced, the Treasury should be refilled, public debt should be reduced, the arrogance of officialdom should be tempered and controlled, and the assistance to foreign lands should be curtailed, lest Rome will become bankrupt. People must again learn to work instead of living on public assistance." - Cicero, 55 BC " We the people are the rightful masters of both Congress and the courts, not to overthrow the Constitution but to overthrow the men who pervert the Constitution." - Abraham Lincoln |
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