Speedometer running slow

[Sierra]

1978 B210 4 speed.

The speedometer has been running slow ever since I purchase this little B.

I running the stock wheels and the right size tires.

Any suggestions on how can I fix this issue?

[datzenmike]

If you bought it used the transmission may have been changed or possibly the rear end. The speedometer drive pinion in the transmission is mated to the rear axle and should be kept together. 

 

A '78 B-210 4 speed should have a 3.70 differential in it from the factory but earlier years and all automatics had 3.889. Possible an earlier 4 speed or an earlier differential was swapped. 

 

You should probably have a 17 tooth pinion gear inside for your year of B-210. If you have an 18 tooth then this is likely the problem and you are reading too slow.

 

If you have a 17 tooth pinion perhaps the rear end was changed to a 3.889 and your are now reading too fast.

[Sierra]

thanks again for your inside datzenmike.

Any markings on the speedometer drive pinion in the transmission, or differential (3.70 or 3.889) that I should be looking for?

I order to have the perfect match?

[datzenmike]

The pinion would have to be removed. (10mm bolt holding a key that align the pinion)

Something like this...

Pull straight out, trans oil may pour out so jack the back up and /or bark downhill. The plastic gears are color coded but count just in case.

 

The differential is more problematic. If removed the ratio is stamped on the crown gear. OR...