Wrong speedo gear?
Thread Starter
Instructor
Joined: Jan 2010
Posts: 153
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From: Buffalo Gap Texas
Wrong speedo gear?
In my 79, I show 80 when GPS tells me I'm actually doing 70. The speedo doesn't bounce at all or show any signs of a worn or broken cable, so I am assuming this car may have been regeared and the speedo gear not corrected when that was done. Would that difference at that speed lead you to believe this could be the case? I have no clue what gear ratio it is currently running, as I just picked it up and noticed the discrepancy on my first trip home. Any other ideas what might be causing the inaccuracy or ideas on what steps I can take to figure out where the error is coming from?
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Remove your speedo cable gear from the transmission and count (or read) the number of teeth on that gear. For the proper correction to be made, you will need a gear with 3 more teeth than what is now on it. That will 'slow down' the rotation of the speedo cable about 10 mph.
Burning Brakes
Joined: Jan 2001
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From: Loir Valley Sarthe, France
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The speedo gears are colour coded to help determine the number of teeth. Corvette Central shows a good range if you search their 68-82 Corvettes site for "Speedometer Drive Gear".
I assume that the speed difference is consistent? If not it may not be a gear issue.
I just replaced the seal and gear on my ’78. It’s an auto with a 3:08 differential and therefore takes a 19 tooth gear, which is natural in colour.
Interestingly, the speedo on the car is absolutely accurate, based on the GPS, to within ½ mile an hour throughout the speed range!
And in my case the job was a pain, due to the full dual exhaust and “fancy” bracket that the exhaust fitter added! But thankfully, with the car lifted on the driver’s side, no trans fluid escaped while I made the swap!
I assume that the speed difference is consistent? If not it may not be a gear issue.
I just replaced the seal and gear on my ’78. It’s an auto with a 3:08 differential and therefore takes a 19 tooth gear, which is natural in colour.
Interestingly, the speedo on the car is absolutely accurate, based on the GPS, to within ½ mile an hour throughout the speed range!
And in my case the job was a pain, due to the full dual exhaust and “fancy” bracket that the exhaust fitter added! But thankfully, with the car lifted on the driver’s side, no trans fluid escaped while I made the swap!
Last edited by Stephen Irons; May 15, 2012 at 04:33 AM.
Thread Starter
Instructor
Joined: Jan 2010
Posts: 153
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From: Buffalo Gap Texas
Great info, guys. As soon as I get back home to the car, I'll give it a look. The wrong speed isn't really the part that's been bothering me, it's that I am racking up mileage at a higher rate!
Advanced
Joined: Jun 2010
Posts: 61
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I'm the opposite issue
My speedo is slow by about 5mph at 40, 7mph at 50 at 10mph at 60 according to the gps. Luckily no speeding tickets before I found out. Speedo is "accurate" around 25-30.
Any suggestions for this situation?
Any suggestions for this situation?
Safety Car
Joined: Sep 2007
Posts: 4,454
Likes: 328
From: Indy Indiana
if your speedometer error is not linear then could be probs in speedo head itself. big job, removing dash, sending in for repair, waiting 6 weeks, then reinstalling...
a slow speedo needs to spin faster at the same car speed, do that by buying a gear with less teeth.
for a cheap test fix, the gear in the tranny may be slipping or worn, but at least try to replace it with a gear with 1 or 2 or 3 less teeth. they are cheap.. buy all 3. more to ship than the cost just about.
a slow speedo needs to spin faster at the same car speed, do that by buying a gear with less teeth.
for a cheap test fix, the gear in the tranny may be slipping or worn, but at least try to replace it with a gear with 1 or 2 or 3 less teeth. they are cheap.. buy all 3. more to ship than the cost just about.
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Thread Starter
Instructor
Joined: Jan 2010
Posts: 153
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From: Buffalo Gap Texas
Finally got around to checking my gear. It's a 19 "natural" color. However, when I checked all the suppliers, the one I need (22), and all the others, are listed as 4 speed tranny gears. Is that correct for my Th350?
Melting Slicks
Joined: Aug 2008
Posts: 2,200
Likes: 3
From: Lehigh county Pennsylvania
4 speed and auto take different plastic 'driven' speedo gears. [the 'drive' gear is pressed onto the trans mainshaft] The vendors sell both. Keep looking for the right listing.
As far as them being 'cheap' goes, I didn't think 10 bucks was exactly cheap. Buying 3 equals 30 bucks. I very much doubt any vendor worth his business license would charge more than 30 bucks shipping for 3 plastic gears. I bought the one I needed from this forum's sponsor, Zip Products.
As far as them being 'cheap' goes, I didn't think 10 bucks was exactly cheap. Buying 3 equals 30 bucks. I very much doubt any vendor worth his business license would charge more than 30 bucks shipping for 3 plastic gears. I bought the one I needed from this forum's sponsor, Zip Products.
Last edited by 69 Chevy; Aug 4, 2012 at 10:56 AM.
Racer
Joined: Sep 2007
Posts: 288
Likes: 1
I just checked. I didn't have a 19 tooth gear but did have a 18, 20 and 22. The 22 is a bit lager in diameter I don't think it would work.
Good luck. I am needing a GM 31 tooth gear
Was just thinking. Plastic you could file down. May need to reshape the teeth.
Good luck. I am needing a GM 31 tooth gear
Was just thinking. Plastic you could file down. May need to reshape the teeth.
Last edited by cgc; Aug 4, 2012 at 11:34 AM.
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
If your present gear is 19 tooth, then you do need a 22 tooth gear. I've changed the gear in my TH350 trans sevaral times and I believe the gears for the 4 speed and the TH350 are the same.
Thread Starter
Instructor
Joined: Jan 2010
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From: Buffalo Gap Texas
This one is listed under both manual and auto, even though it says manual in the title. Same part number in both sections and in the description is says it works for both.
http://www.zip-corvette.com/ProductDetails/ProductDetails.aspx?pid={6bb0a08a-3f50-4a16-b270-9946f840ecf3}&gid={fc1222b0-16ab-4afb-a2fe-f24041cb4ab4}&GroupName=Auto+Trans&pname =57-81+Manual+Speedometer+Driven+Gear+-+22+Tooth+-+Green&Referer=&Alias=&ptct=SGR-SR&CTitle=&
http://www.zip-corvette.com/ProductDetails/ProductDetails.aspx?pid={6bb0a08a-3f50-4a16-b270-9946f840ecf3}&gid={fc1222b0-16ab-4afb-a2fe-f24041cb4ab4}&GroupName=Auto+Trans&pname =57-81+Manual+Speedometer+Driven+Gear+-+22+Tooth+-+Green&Referer=&Alias=&ptct=SGR-SR&CTitle=&