Speedometer Error
Thread Starter
Cruising
Joined: Apr 2013
Posts: 10
Likes: 2
Speedometer Error
I just bought a 72 C3 and the spedometer is way off - when it indicates 55 you are doing 80, an indicated 45 is actually 63. A Corvette is sort of asking for a ticket anyway, but this seals the deal. Any advice on how I can get it corrected, especially without having it look like I tampered withthe mileage (it only has 37,000)
Safety Car
Joined: Sep 2007
Posts: 4,443
Likes: 326
From: Indy Indiana
get you a different speedometer driven gear. it sits on the tranny where the cable plugs in at.
when you are driving faster than indicated then your cable is not spinning fast enough. you speed it up by dropping the number of teeth in the driven gear..
the gears are cheap and easy to replace with the right pretzel hands up in there.
get yours out and count the teeth and start out with another one with 4 or 5 less teeth. adjust up or down accordingly.
when you are driving faster than indicated then your cable is not spinning fast enough. you speed it up by dropping the number of teeth in the driven gear..
the gears are cheap and easy to replace with the right pretzel hands up in there.
get yours out and count the teeth and start out with another one with 4 or 5 less teeth. adjust up or down accordingly.
Race Director
Joined: Sep 1999
Posts: 11,358
Likes: 379
From: Plano TX
First check the odometer. Use a GPS and drive for 10 miles or so and compare how many miles the GPS reads vs the odo. It is possible for the speedo head to become demagnetized with age and the speed will read low while the odo reads correct.
Thread Starter
Cruising
Joined: Apr 2013
Posts: 10
Likes: 2
Wow - great questions and very prompt replies. From what I can see on the tank sticker (very faded) it is a 3.08 rear end. My recollection is that I was seeing about 3000 rpm in 4th gear when I had the 55 indicated/80 actual (from the GPS) split. It would be ideal if the gear was reachable from the top side as lifting is pretty challenging with side pipes. As it seems to be a trial and error thing, it would require multiple lifts to get it right. That's OK though - no one promised it would be easy. Unfortunately, I am seperated from the car for a couple of months, but would like to have a plan in hand when we are reunited and I can address the condition. That's really the only problem with it - the original clock even works. There is a small seatbelt issue that I may post separately, but it is just too much fun to do anything but drive.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
You need to follow "Zwede's" advice. Use your GPS to check the accuracy of the ODOMETER (mileage indicator) first. If the 'distance' measurement portion of the speedometer is relatively accurate, but the speed indicator is NOT, then the problem is in the speedo head and it will have to be repaired or replaced to fix your problem.
If, however, the odometer reads "short" by the same amount (percentage wise) as the speedo readings, then the problem can be solved by swapping out the speedo drive gearset in the transmission tail housing.
I'm guessing, but with a speedo reading that far off, your problem is MOST likely to be in the speedo head. But, check out the odometer first, so that you are working in the right area to solve your problem.
If, however, the odometer reads "short" by the same amount (percentage wise) as the speedo readings, then the problem can be solved by swapping out the speedo drive gearset in the transmission tail housing.
I'm guessing, but with a speedo reading that far off, your problem is MOST likely to be in the speedo head. But, check out the odometer first, so that you are working in the right area to solve your problem.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
MY problem a few years ago was with it OVER indicating, and the tach also...but my car is highly modified, nothing stock, from wheels to roof....
the tire sites said so many RPM for so much speed with given ratios and tire diameters.....well, they are RONG!!!
my car was saying 40 and I was doing maybe 33, then at 80 indicated I was doing like 68 or something.....
put in a driven gear to drop the readings on the speedo, and sure enough....dead nutz on with Garmin, up to 70 mph....at 80 it was really only 78, at 100 it was only really 94, ran outta room, that tells me the linearity of the speedos is done that way, most likely for safety.....
tachs over indicate also by several hundred RPM, in the upper regions....
the tire sites said so many RPM for so much speed with given ratios and tire diameters.....well, they are RONG!!!
my car was saying 40 and I was doing maybe 33, then at 80 indicated I was doing like 68 or something.....
put in a driven gear to drop the readings on the speedo, and sure enough....dead nutz on with Garmin, up to 70 mph....at 80 it was really only 78, at 100 it was only really 94, ran outta room, that tells me the linearity of the speedos is done that way, most likely for safety.....
tachs over indicate also by several hundred RPM, in the upper regions....
