Speedometer adjustment

[Tkmadone]

I have checked the odometer and it is very accurate, but the speed is off about 10% at 65 mph. So it shows 60 but I'm actually doing 66.

So I know I don't need to change the gears in the transmission but is it possible to calibrate just the speed?

This is in a 1967 C20 2wd.

Thanks!

[geezer#99]

If your speedo is off 10% then your odometer is too!
To correct it you need to change the driven gear or use a ratio adaptor.
Look here.
http://www.transmissioncenter.net/sp...n_______va.htm

[68GMCCustom]

what trans? should be able to change the speedo gears in the trans?

[Tkmadone]

I verified with GPS that the odometer is only off .1 miles over 50 so the gears are correct, it's just the magnetic part of the speedometer that needs adjustment.

The transmission is a TH400.

[Tkmadone]

So it turns out you can adjust your indicated speed relative to your odometer.

I took the speedometer out, and if you look at the shaft where it goes into the cup where the magnets are, there is a metal piece that bridges the gap and the spring that returns the needle is attached to it. All you have to do is rotate that piece of metal, either to decrease the spring load, which increases indicated speed, or increase the spring load, which decreases the indicated speed.

I use a drill with a small screwdriver for a bit and used it to spin the speedometer before and after adjustment. Just adjust until the speed you want to show does and it should be fine.

Anyway, just thought this might help somebody in the future. Remember, this is only necessary if your odometer is accurate but your speed isn't.

[hotrodder]

Thank you for the info on adjusting the speedometer. Mine is off just a little, so I am sure this will correct mine. I was about to start swapping the gears around,and that would not have solved the problem. Nice to have people on the forum that take the time to help each other out.

[engineer_gregh]

Great post!

[Longhorn 70]

Tk, you didn't happen to take pics did you?

[bollybib]

Good info here, thanks.

[Yooper truck]

A couple questions:

Do you remember which way to turn to speed up or slow down the speedometer?

About how many turns did you need to adjust your 10 mph?

Thanks for posting!

[Tkmadone]

I didn't take any pics so I stole this one from OldGold's trip odometer post, I hope he won't mind. It shows the adjustment bar pretty well and I added text to show it.

To add 4 mph I only had to move it about 1/8" clockwise (looking from the back). It's pretty sensitive so you just rotate it a little then check, rinse and repeat until you get what you think you need.