Speedometer Correction

[Ralph E.]

Re: Speedometer Correction

Drove along side my wifes car. She was reading 65 mph on her speedo and I was reading 50 mph on mine. 15/65 is 23% so I guess I need to check my driven gear and replace with a gear with 23% fewer teeth. I think I'll also try and check the rear axle ratio.

[Ralph E.]

Re: Speedometer Correction

Drove along side my wifes car. She was reading 65 mph on her speedo and I was reading 50 mph on mine. 15/65 is 23% so I guess I need to check my driven gear and replace with a gear with 23% fewer teeth. I think I'll also try and check the rear axle ratio.

[Duke W.]

Re: Speedometer Correction

Counting turns on the driveshaft/halfshafts to verify the axle ratio is a good idea. If it's a positraction in good shape both wheels will turn, so the counting is easy.

While it's up in the air, pull the speedometer driven gear out and count the teeth and note the color. Also, if at all possible count the teeth and note the color of the drive gear inside the box. You might have some sort of oddball combination.

Duke

[Duke W.]

Re: Speedometer Correction

Counting turns on the driveshaft/halfshafts to verify the axle ratio is a good idea. If it's a positraction in good shape both wheels will turn, so the counting is easy.

While it's up in the air, pull the speedometer driven gear out and count the teeth and note the color. Also, if at all possible count the teeth and note the color of the drive gear inside the box. You might have some sort of oddball combination.

Duke

[Ralph E.]

Re: Speedometer Correction

Thanks, I didn't realize that both gears should be the same color. Can't wait to check this out!

[Ralph E.]

Re: Speedometer Correction

Thanks, I didn't realize that both gears should be the same color. Can't wait to check this out!

[Duke W.]

Re: Speedometer Correction

The drive and driven gears are not necessarity the same color. Driven gears are different colors to aid in identification. I beleive drive gears are color coded, too. That's why I suggested you note the color of BOTH the drive and driven gears. They are probably different. Assuming you have a manual transmission your drive gear should be eight teeth. I don't remember off hand what color it should be, but someone should be able to verify the color for the correct eight tooth drive gear.

Duke

[Duke W.]

Re: Speedometer Correction

The drive and driven gears are not necessarity the same color. Driven gears are different colors to aid in identification. I beleive drive gears are color coded, too. That's why I suggested you note the color of BOTH the drive and driven gears. They are probably different. Assuming you have a manual transmission your drive gear should be eight teeth. I don't remember off hand what color it should be, but someone should be able to verify the color for the correct eight tooth drive gear.

Duke

[Ralph E.]

Re: Speedometer Correction

Took out the driven gear, it's green with 22 teeth. I didn't check the rear ratio. I should have but I wanted to go for a ride.

[Ralph E.]

Re: Speedometer Correction

Took out the driven gear, it's green with 22 teeth. I didn't check the rear ratio. I should have but I wanted to go for a ride.

[Ralph E.]

Re: Speedometer Correction

I went to changed the speedo driven gear from the GREEN-22 teeth to BLUE-20 teeth and found that the diameters of the two gears are different. The GREEN gear is slightly larger. I also check my rear axle, 3:36 Positraction. I think I just discovered that my transmission is from a 1965 Corvette, my car is a 1967. What can I say? I check with my local Chevy dealer and he tells me the same part will fit all midyears. I believe the tranny may be from a car with a 3:73 rear. What can I do to fix this mess??

[Ralph E.]

Re: Speedometer Correction

I went to changed the speedo driven gear from the GREEN-22 teeth to BLUE-20 teeth and found that the diameters of the two gears are different. The GREEN gear is slightly larger. I also check my rear axle, 3:36 Positraction. I think I just discovered that my transmission is from a 1965 Corvette, my car is a 1967. What can I say? I check with my local Chevy dealer and he tells me the same part will fit all midyears. I believe the tranny may be from a car with a 3:73 rear. What can I do to fix this mess??

[Joe L.]

Re: Speedometer Correction

Ralph------

Well, after a lot of posts to this question we finally got to the point where you've provided enough information for me to work with. Here's the deal:

Given the fact that you have tires with essentially the same OD (or, revs per mile) as stock, we can take that out of the "equation". All that really matters, then, is your rear axle ratio in determining what speedo gears you need. If your car is, indeed, a 3.36:1 ratio, then you require a speedometer DRIVE gear of GM #3708144 and a speedometer DRIVEN gear of GM #3987920. This is a blue gear of 20 tooth count.

If the DRIVEN gear that came out of the transmission was a green gear of 22 tooth count, that would be consistent with a transmission originally installed in a car with a 3.73:1 ratio. However, the 22 tooth green gear is compatible only with speedometer DRIVE gear GM #3708145. So, that means that you have the GM #3708145 DRIVE gear installed in your transmission and that gear is NOT COMPATIBLE with the 20 tooth blue gear that you need. Thus, you have 2 choices:

1) remove the transmission, partially disassemble it, and replace the speedo DRIVE gear on the mainshaft with the 3708144 gear. The problem, besides the work involved, is that this gear is GM-discontinued, so you'll have to obtain a used one or an aftermarket unit if one is available. Both the GM #3708144 and GM #3708145 gears are STEEL gears of 8 teeth, but their diameters are different.

2) keep your existing gearing and obtain a speedometer adapter to convert the output of the gear system to an accurate one. If you elect this option, I would suggest that you take the car to a competent speedometer service shop and have them set up an adapter to provide the right correction. I THINK that the adapter that you would need to make the conversion would be GM #15635469 (which is a 1.1111 ratio adapter), but this adapter is an expensive part ($50.00 GM list) and you might not want to experiment with what I THINK might work.

[Joe L.]

Re: Speedometer Correction

Ralph------

Well, after a lot of posts to this question we finally got to the point where you've provided enough information for me to work with. Here's the deal:

Given the fact that you have tires with essentially the same OD (or, revs per mile) as stock, we can take that out of the "equation". All that really matters, then, is your rear axle ratio in determining what speedo gears you need. If your car is, indeed, a 3.36:1 ratio, then you require a speedometer DRIVE gear of GM #3708144 and a speedometer DRIVEN gear of GM #3987920. This is a blue gear of 20 tooth count.

If the DRIVEN gear that came out of the transmission was a green gear of 22 tooth count, that would be consistent with a transmission originally installed in a car with a 3.73:1 ratio. However, the 22 tooth green gear is compatible only with speedometer DRIVE gear GM #3708145. So, that means that you have the GM #3708145 DRIVE gear installed in your transmission and that gear is NOT COMPATIBLE with the 20 tooth blue gear that you need. Thus, you have 2 choices:

1) remove the transmission, partially disassemble it, and replace the speedo DRIVE gear on the mainshaft with the 3708144 gear. The problem, besides the work involved, is that this gear is GM-discontinued, so you'll have to obtain a used one or an aftermarket unit if one is available. Both the GM #3708144 and GM #3708145 gears are STEEL gears of 8 teeth, but their diameters are different.

2) keep your existing gearing and obtain a speedometer adapter to convert the output of the gear system to an accurate one. If you elect this option, I would suggest that you take the car to a competent speedometer service shop and have them set up an adapter to provide the right correction. I THINK that the adapter that you would need to make the conversion would be GM #15635469 (which is a 1.1111 ratio adapter), but this adapter is an expensive part ($50.00 GM list) and you might not want to experiment with what I THINK might work.

[Ralph E.]

Re: Speedometer Correction

Keeping the existing gear and obtaining a speedometer adapter appears to be a good solution. Any thoughts where to find one. Here in Connecticut we don't have any speedometer service places.