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WVZR-1 · 07-09-2009

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!

07-09-2009	#2
WVZR-1 Join Date: Oct 2004 Posts: 1,884	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.