Speedometer gear
Thread Starter
Intermediate
Joined: Jun 2019
Posts: 32
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Speedometer gear
I bought a 1980 L82 six years ago that at the time was stock with a 350 auto trans. I have the same 255 60/R15 tires. At the time the speedometer ran about 5mph too fast in the 40 to 60mph range. I have since rebuilt the rear end and dropped to a 355 rear gear. Now the speedometer reads about 15mph to fast in the 40 to 60mph range. I don't know how many teeth the stock gear in the trans had but wondered if I need more or less teeth. I also needed to know what additional parts you need when changing out the gear. Last, how hard is it to get to and how hard to do. Is there a cross member you have to remove to get to it, especially if you have to try several gears to get it right. Was curious if anyone can tell me the original rear end gear size. I was guessing about a 308. Thanks for any help or recommendations. Knowing close to how many teeth were on the original gear would be helpful for a baseline. I know the gears are different colors and an idea number of teeth to go to to try and get close would be very helpful, thanks
Last edited by alan600; Apr 15, 2024 at 07:59 PM. Reason: Added an additional question
Drifting
Joined: Aug 2016
Posts: 1,777
Likes: 924
From: Arizona
I bought a 1980 L82 six years ago that at the time was stock with a 350 auto trans. I have the same 255 60/R15 tires. At the time the speedometer ran about 5mph too fast in the 40 to 60mph range. I have since rebuilt the rear end and dropped to a 355 rear gear. Now the speedometer reads about 15mph to fast in the 40 to 60mph range. I don't know how many teeth the stock gear in the trans had but wondered if I need more or less teeth. I also needed to know what additional parts you need when changing out the gear. Last, how hard is it to get to and how hard to do. Is there a cross member you have to remove to get to it, especially if you have to try several gears to get it right. Was curious if anyone can tell me the original rear end gear size. I was guessing about a 308. Thanks for any help or recommendations. Knowing close to how many teeth were on the original gear would be helpful for a baseline. I know the gears are different colors and an idea number of teeth to go to to try and get close would be very helpful, thanks
I went from th350 to 2004r and needed to replace them both, I needed to drop the pan but I don't believe you need to with the th350, others might know better
Drifting
Joined: Aug 2019
Posts: 1,508
Likes: 495
You need one less tooth on the counter shaft/ drive gear and 3 more teeth on the driven gear
one tooth on the drive gear is 10 mph , more teeth makes the Speedo read faster , less teeth slower
driven gear at the cable is one tooth equals 2 mph , more teeth slow down the Speedo , less teeth speed it up .
They sell gear kits on eBay and you need to remove the tail shaft housing to access the drive gear , the driven gear just needs the speedometer housing removed from the tail shaft
one tooth on the drive gear is 10 mph , more teeth makes the Speedo read faster , less teeth slower
driven gear at the cable is one tooth equals 2 mph , more teeth slow down the Speedo , less teeth speed it up .
They sell gear kits on eBay and you need to remove the tail shaft housing to access the drive gear , the driven gear just needs the speedometer housing removed from the tail shaft
Thread Starter
Intermediate
Joined: Jun 2019
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Thank you. I need to do a little studying to totally understand but this helps tremendously. I know my rear end gear and tire size, I just need to know how to put this all together. I apppreciate all the help I can get as mine is so far off it is driving me crazy instead of me just driving.
Drifting
Joined: Aug 2019
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Melting Slicks
Joined: Jun 2014
Posts: 2,226
Likes: 497
From: NJ
Check out Stewart- Warner, the gauge people, they make all kinds of speedometer correction gear sets to either increase or decrease the MPH. They attach to the speedo housing on the tail shaft. It has interchangeable gear sets to match just about any offset. And 1000 RPM of the speedo cable equals 60MPH on the speedometer, that is the baseline. Given that, you need to slow down the RPM of your speedo cable. It's been about 25 years since I used them and not sure if they still exist but, it's worth a shot, since you don't have to open the tranny to do it.
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Intermediate
Joined: Aug 2023
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From: Virginia
I took the easy way out on my 74. The driven gear was bad and I did not want to go to the trouble of trying to drop the crossmember and try to remove the housing. My solution was the Jegs GPS Conversion. The box mounted under the hood and already having the two piece cable, installation was fast and easy. It works flawlessly and is extremely accurate.