Speedometer Conversion
Thread Starter
Heel & Toe
Joined: Apr 2008
Posts: 17
Likes: 0
Speedometer Conversion
I bought a good working 140mph speedometer out of a 1979 Corvette. I intend to replace the 85mph speedometer in my 81 Corvette. Will this conversion work? Do I need to change anything in the transmission? Thanks.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Should work fine, except for the PITA labor involved. Converted my '78 140 to a 165 years ago without any calibration or mods necessary. FWIW, once you've changed the head I'd suggest you disclaim the odo reading's accuracy (on the title or bill of sale) should you ever sell the car.
Melting Slicks
Joined: Apr 2001
Posts: 3,399
Likes: 278
From: Olmsted Falls, Ohio
Should work fine, except for the PITA labor involved. Converted my '78 140 to a 165 years ago without any calibration or mods necessary. FWIW, once you've changed the head I'd suggest you disclaim the odo reading's accuracy (on the title or bill of sale) should you ever sell the car.
Could he change out the transmission gear to get accurate mileage?
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Assuming the existing one is reading correctly now, unlesss the new speedo is only a reface it should already be properly calibrated and give correct MPH without changing the transmission's speedo output gearing, which also means the odometer will accumulate mileage accurately too. It's the total mileage that will never be accurate without running it up to match what's already on the clock before installation.
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Last edited by TheSkunkWorks; Jun 25, 2012 at 11:42 AM.
Thread Starter
Heel & Toe
Joined: Apr 2008
Posts: 17
Likes: 0
Ok thanks! My old speedometer was broke and didn't function so the mileage was already off when I got the car. It has been through a substantial renovation since then. I took the 140mph speedometer apart and reset the odometer to zero. I plan to put the dash and gauges in the car next week.
Race Director
Joined: Apr 2007
Posts: 11,150
Likes: 890
From: South Western Ontario
It seems to me all GM mechanical speedos expect 1000 revolutions per mile or they'll read 60mph when the input is turning 1000rpm. So, it will be fine if nothing in the driveline or rear tire size changed. If you did change something, then use one of the online speedo gear selection tools.
Melting Slicks
Joined: Apr 2001
Posts: 3,399
Likes: 278
From: Olmsted Falls, Ohio
Sorry, I stated my previous question wrong. I did not mean mileage I meant speed.
If I'm not mistaken on the older speedos you can remove the back and cover where the number rollers are and manually reset the numbers.
If I'm not mistaken on the older speedos you can remove the back and cover where the number rollers are and manually reset the numbers.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
FWIW, if your car has cruise control, could very well be the transducer went and in the process broke a cable, resulting in a non-functioning speedo. So, I'd check for that. And, I'd hang onto the old head in case having it on hand helps you sell down the road.
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