Speedometer adjustment

[Antz81]

Can the speedometer zero point be adjusted in the same way as the tachometer? Mine reads about 10% faster than my actual speed.

 

[bill81vette]

You may have to change the gear that drives the speedo in the tranny...perhaps it's the wrong one.

Sent from my XT1080 using Tapatalk

 

[Tom Bryant]

No adjustment on the actual speedometer other than having a speedometer service shop calibrate it. That requires removing it from the instrument panel. If it's reading faster than actual speed you need to slow down the cable speed. Changing to a speedometer driven gear that has more teeth will do that. This would be the plastic gear on the lower end of the speedo cable. First you will need to pull off the cable and the adapter going into the the transmission to see what tooth count gear you have in there now.

For a 4 speed car with the original 2.72 rear end gear ratio and either 225/70 or optional 255/60 tire size, you should have a natural colored gear with 19 teeth, GM part number 3987919. An automatic equipped car with standard 2.87 rear gear would have a 20 tooth driven gear, blue in color, GM part number 3987920. This gear would slow the cable some. Not sure if it would be enough. Also there is a 21 and a 22 tooth gear listed for 1976-1978 cars that would slow it even more. They are GM part numbers 3987921(Red) and 3987922(silver) respectively.

If you have changed tire size to a shorter overall diameter or changed the rear gear to a higher numerically gear ratio, such as a 3.08, that will also increase the cable speed. If nothing has been changed it must simply be a calibration problem in the speedometer head. The easiest way to correct it would be to order all three of the larger driven gears in 20, 21, and 22 tooth count and swap them out until you get as close to accurate as possible. If you can get close that would be a lot simpler than pulling out the speedometer.

It seems like we used to figure 2.5 mph change per tooth but don't hold me to that. It has been a long time since I was in GM parts.

Tom

 

[Vettehead Mikey]

Can the speedometer zero point be adjusted in the same way as the tachometer? Mine reads about 10% faster than my actual speed.

Does the odometer record accurately?

If 'no' and it's off by the same 10%, change the driven gear in the transmission.

If 'yes', the odo is accurate, the speedo will need to be sent out for calibration.

 

[Antz81]

Does the odometer record accurately?

If 'no' and it's off by the same 10%, change the driven gear in the transmission.

If 'yes', the odo is accurate, the speedo will need to be sent out for calibration.

Never paid much attention to the odometer so I'll have to check that.

Also made a small error in the OP. My speedo reads slower than my actual speed. So if I do need to change the gears then I want to decrease the number of teeth.

 

[Tom Bryant]

Yes. But now you have a problem. If your gear is the original 19 tooth you only have one smaller available. That 's an 18 tooth. I guess the next thing to do is find out what gear you have. By the way, what tires size do you have on the rear?

Tom

 

[Antz81]

Yes. But now you have a problem. If your gear is the original 19 tooth you only have one smaller available. That 's an 18 tooth. I guess the next thing to do is find out what gear you have. By the way, what tires size do you have on the rear?

Tom

I guess that could be and issue.
Current tires are 245/60r15. (Put on by a previous owner)
I will be changing them back to 255 when I need to replace them. The larger overall diameter of the 255 won't help if I do have the original gear.

 

[Antz81]

Looks like bubba needed a speedo gear and any would do. I have the red 21 tooth gear.
By my calculations changing back to the correct 19 tooth gear should correct the problem. May try an 18 as well just to check what it shows.

 

[Tom Bryant]

It looks like you found the problem.

Tom

 

[Antz81]

New speedo gear arrive a couple of days ago. Fitted it today and took it for a test drive: It now reads about 2% faster than my actual speed. Once i get the correct 255/60R15s back on it that will go down (to about 0.3% if my calculations are correct.) That makes it the most accurate speedo i have seen in any car.

 

[Tom Bryant]

That's good news.