working out what diff ratio is in the car

[pad4]

as my speedo is about 12 mph out in the 8 port and im not sure what the diff ratio is in the box is there any way i can work out whats what to get my speedo calibrated - all i can say its a good job the police chap was stuffing his face as i passed him earlier this morning - might be a stupid question but i just cant get my head round it today

pad

[abri]

Somewhere on this forum is a thread that has info on this. Unfortunately I can't find it.

What you do is remove the speedo cable from the back of the speedo and put it down in a way that you can see the rotation on the inner cable. Mark a distance of 10 meters on flat ground and push the car for the distance while counting the number of rotations of the cable.

Post the number of rotations you counted over 10 meters and someone will be able to tell you what ratio would give you that figure.

[GraemeC]

Counting the speedo cable revs won't give you the diff ratio, but would tell you if the speedo is mismatched.

To get diff ratio:
Car in 4th, plugs out. Mark tyre at bottom most point and make sure you can clearly see a mark on the bottom pulley.
With an assistant or two push the car forward one full revolution of the wheel and count how many revolutions of the crank pulley this results in.
(This assumes 4th gear ration and the drop gears are both 1:1)

3.6 revs of crank pulley = 3.6:1 diff ratio.
For more accuracy do 10 wheel revolutions. May be easier at this point to jack one whell off the floor rather than push the car though!

Once you know the diff ratio and the TPM of the speedo head you can select the right speedo drive gears to give the best readout.

[cheleker]

I do a variaton of GraemeC's suggestion. I find it easier to figure partial turns of the tire than the crank pulley. Mark the tire with chalk into quarters (8ths, if you like) and count complete revolutions of the pulley. Ten is a good number. Divide by tire revolutions. Repeat.