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Orange Hash Marks On Speedometer


Poloace
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The marks on the '76s, with a 3.91 diff, are in the same place as they are on the '75s, with a 3.64 diff, but the big difference is that on the '76s the speedo has an overlay that has been rotated so that, for example, straight up on the speedo is 63 mph on the '75s but only 59 mph on the '76s.

 

Driving side-by-side down the freeway at the same speed (say 75 mph) my wife's '76 is turning approx. 400 rpm more that my '75 with the 3.64 rear end.  Makes her car a little noisier "at speed" on the freeway. 

 

Bob Napier

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The marks on the '76s, with a 3.91 diff, are in the same place as they are on the '75s, with a 3.64 diff, but the big difference is that on the '76s the speedo has an overlay that has been rotated so that, for example, straight up on the speedo is 63 mph on the '75s but only 59 mph on the '76s.

Driving side-by-side down the freeway at the same speed (say 75 mph) my wife's '76 is turning approx. 400 rpm more that my '75 with the 3.64 rear end. Makes her car a little noisier "at speed" on the freeway.

Bob Napier

I certainly agree with this but also note that my '76's speedometer (which is the original 3/76 unit, marked W=1.393, as appropriate for the 3.90 diff) remains optimistic -- i.e., readout exceeds speed as measured by GPS -- by approximately 8%. I have always run the original 165HR-13 XAs tires. I didn't realize there was such inaccuracy until I first used a GPS unit in the car and was shocked by it. But upon discovering this latent optimism, I finally understood why my bone stock 2002 indicated a steady 115 mph from Rochester to Syracuse, New York on a foot-to-the-floor jaunt during my hoodlum youth period (shave 8% and you've got the actual speed).

Does anyone else, running a stock 3.90 diff (the '76-only 2002 diff is a bona fide 3.90, whereas the 3.91, found in e21s, is a bona fide 3.91 -- even Korman didn't believe me until they counted the gear teeth), correct 1.393 speedometer, and original size tires have this sort of speedometer error? There was a fuss in late '75 -- documented by notes in the Roundel Magazine -- about early '76-model speedometers not having the overlay, but it was allegedly corrected many months before my April '76 car was built. Do I need to crack open my speedometer and check for a missing overlay, or is my 8% error pretty much the norm?

Sorry -- I think I just hijacked the thread!

Thanks and regards,

Steve

EDIT: Bob, you solved my mystery! I'm missing the overlay as my speedometer shows 63 mph straight up, not 59 mph! I guess I should have read more carefully before hijacking the thread!

Edited by Conserv
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Conserv, is your odometer function off as well or only the speed?  The speedo gearing may be correct but the needle spring not correctly tensioned.

I haven't a clue about the odometer so I obviously must check! Finding the face overlay that I now realize is missing from my speedometer (see my EDIT above) seems like it would largely correct my unit's error (it, reportedly, corrects for a 7% error while my car's error is approximately 8%). But, before I discovered this -- as in today -- I assumed that I would simply document the speedometer's errors and send it to North Hollywood Speedometer for calibration. Is calibration likely to represent an improvement over a new face overlay?

Thanks,

Steve

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