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Oct 2, 2019 | 07:30 PM

oldyella

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Engine RPM

What would be the expected RPM at 60mph in a 1968 corvette with L79, stock 4 speed in 4th gear, with 3:70 rearend and 26.6 inch tall tires? I think I covered all the parameters. I haven't checked anything yet but was told it was a 3:70 gear ratio and was original to the car. I checked the speedometer once with gps and it appeared to be close. I am seeing 2500 at 60. Seems low to me.
Thanks in advance.

Oct 2, 2019 | 08:00 PM

DC3

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From: Lubbock Texas

I'm calculating 2804 RPMs at 60 MPH.

DC

Oct 3, 2019 | 12:20 AM

oldyella

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Thanks

Oct 3, 2019 | 11:14 AM

lars

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From: At my Bar drinking and wrenching in Lafayette Colorado

Quote:

Originally Posted by DC3

I'm calculating 2804 RPMs at 60 MPH.

DC

I'm getting 2805 rpm...

Quote:

Originally Posted by oldyella

What would be the expected RPM at 60mph in a 1968 corvette with L79, stock 4 speed in 4th gear, with 3:70 rearend and 26.6 inch tall tires? I am seeing 2500 at 60. Seems low to me.Thanks in advance.

If your numbers are correct and you're getting 2500 rpm at 60, you have a 3.30:1 rear end ratio... You might want to check your tach against another tach and see if it's reading correctly: If it's right, then you don't have a 3.70 rear end.

Lars

Last edited by lars; Oct 3, 2019 at 11:19 AM.

Oct 3, 2019 | 01:08 PM

cv67

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From: altered state

St. Jude Donor '05

http://wallaceracing.com/calc-gear-tire-rpm-mph.php

Oct 3, 2019 | 03:48 PM

oldyella

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Contacted the previous owner since posting this and he says he had the rearend apart and counted teeth to arrive at the 3:70 number. So, based on the numbers suggested by you all from my parameters, looks like I do need to check tach and speedometer again. Thanks

Oct 3, 2019 | 04:31 PM

cv67

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From: altered state

St. Jude Donor '05

Celphones can check this stuff nowadays with GPS. Kinda neat.
Saves me a bunch from buying a converter box.

Oct 4, 2019 | 11:53 AM

oldyella

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I have an old dwell/tach that says the tach is reading low. Since this is a direct drive tach, what could be wrong that would cause it to read low? This question is assuming the dwell/tach is reading correctly. The tach moves smoothly as rpm is increased.