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WVZR-1 · 07-10-2009

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.

07-10-2009	#7
WVZR-1 Join Date: Oct 2004 Posts: 1,884	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.