[C1] speedo gears replacement
Thread Starter
Heel & Toe
Joined: Feb 2015
Posts: 20
Likes: 0
speedo gears replacement
My '56 corvette has a th350 transmission behind a 283 ci motor. The car has a 3.55 final drive ratio and has 6.70 sized tires. The problem is that there is about significant error in MPH at various speeds. For example, at 60 MPH (verified by GPS), the original speedo registers approximately 80 MPH. Whereas, at 30 MPH, the speedo registers approx. 40 MPH. I would have guest that the error would have been linear over different speeds. Given this amount of error, I will probably have to change out the entire gear set (drive and driver gears). There does not seem to be a lot of room to remove the tail housing to get at the drive gear while still in the car, given the "X" frame interference. Has anyone done this job before. Or would it be best to get a speed adapter instead and save a lot of work. With that in mind, does a speed adapter unit come with a certain amount of adjustability or does it come with a fixed ratio? Thanks.
Racer
Joined: May 2015
Posts: 277
Likes: 31
From: Park Ridge NJ
I have a 62 that I installed a th350 in last year, I inadvertently installed a slip yoke that was too long, it hit and broke the small metal clip that holds the drive gear on the output shaft of the TH350. Using a long extension and going in from the back I was able to remove the four bolts holding the tail housing cover on and slide it back far enough to replace the gear but you will not physically be able to remove the tail housing from the car. I might suggest to first determine how many teeth your driven gear is and see if you can swap it to see how that helps.
Drifting
Joined: Oct 2001
Posts: 1,550
Likes: 652
From: Richmond VA
The ratio between 80 and 60, and 40 and 30 are the same. In both cases you speedometer is 33% off. I had the same issue in my 62 and to save all the headaches of changing the gear, I just bought a ratio adapter. They are not expensive and are simple to install. Disconnect the cable at the transmission, connect the adapter to the transmission and reconnect the cable to the adapter and you're done. Actually just takes minutes.
Race Director
Joined: Apr 2015
Posts: 17,571
Likes: 3,460
From: Fresno California
I used a similar adaptor (small transmission box) on my old '83 Toyota 4x4 which has run oversized tires all it's life. Did that about 20 years ago and now have a dead-on speedometer. Took about 5 minutes to install it.
Thread Starter
Heel & Toe
Joined: Feb 2015
Posts: 20
Likes: 0
In my situation, changing the driven gear will not fix the problem...too much of a speed error. I would need to replace the driver gear, as well. When you fixed your gear, you had to remove the transmission support bracket to slide the tail housing rearward, correct. Not a lot of wiggle room.
Racer
Joined: May 2015
Posts: 277
Likes: 31
From: Park Ridge NJ
In my situation, changing the driven gear will not fix the problem...too much of a speed error. I would need to replace the driver gear, as well. When you fixed your gear, you had to remove the transmission support bracket to slide the tail housing rearward, correct. Not a lot of wiggle room.
Drifting
Joined: Oct 2001
Posts: 1,550
Likes: 652
From: Richmond VA
I'll dig through my stuff to see where i bought it. You have to tell them how much error you have. They ask me to check the speedometer against the GPS at several speeds so they could supply the correct adapter. There are also adjustable adapters but they are more expensive and not necessary unless you planned to change the rear ratio at some point.
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Thread Starter
Heel & Toe
Joined: Feb 2015
Posts: 20
Likes: 0
Thanks for the contact. I did inquire about the speedo gear adapter this morning. He mentioned that I would need to give him the speedo readings for 40, 50, and 60 MPH. BTW, the cost is now $88. I noticed that the unit is approx. 8x8x6 inches per their web site. Did you have any clearance issues installing the adapter? Are you please with it's performance? Thanks again.
Drifting
Joined: Oct 2001
Posts: 1,550
Likes: 652
From: Richmond VA
Thanks for the contact. I did inquire about the speedo gear adapter this morning. He mentioned that I would need to give him the speedo readings for 40, 50, and 60 MPH. BTW, the cost is now $88. I noticed that the unit is approx. 8x8x6 inches per their web site. Did you have any clearance issues installing the adapter? Are you please with it's performance? Thanks again.
Drifting
Joined: Oct 2001
Posts: 1,550
Likes: 652
From: Richmond VA
Thanks for the contact. I did inquire about the speedo gear adapter this morning. He mentioned that I would need to give him the speedo readings for 40, 50, and 60 MPH. BTW, the cost is now $88. I noticed that the unit is approx. 8x8x6 inches per their web site. Did you have any clearance issues installing the adapter? Are you please with it's performance? Thanks again.
Actually no cable I have a 54RA2. It fit fine in my 62. I believe the dimension is about twice as big as mine. I would give them a call to verify and ask about the fit in your particular car.
Advanced
Joined: Jun 2013
Posts: 96
Likes: 18
From: Florham Park, NJ
I'm replacing the 3.36 rear in my '63 with a 3.08. I'm pretty sure I should replace the gear where the cable comes out of the transmission. Does anyone know how many teeth my current gear has? And, what should I replace it with? Thanks!
Team Owner
Joined: Oct 2000
Posts: 38,897
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From: Washington Michigan
