When my truck’s going 55 mph, the speedometer shows 90

Question:

I have tried to do my homework but am still in the dark on speedometers. On my 1970 Chevy C20 car hauler, the speedometer shows an accurate speed to about 25 then gets progressively weirder, showing 90 mph at an actual 55 mph. I have replaced the gauge unit completely and the cable is just a cable (right?). The transmission is a three-speed automatic and is stock so far as I can tell. How do I fix this thing??

Answer:

This is a perplexing situation indeed. My first assumption would be that the speedometer head is calibrated incorrectly but given that you changed the complete gauge and still have the same problem makes it unlikely that the gauge is to blame.

Speedometer inaccuracies can be caused by problems with cable and cable drive or driven gear, but the result is usually the opposite of what you’re experiencing.

If the cable is slipping in the driven gear because the square recess in the gear’s shaft that receives the square end of the cable is rounded off, or if the gear’s teeth are worn, slippage can occur at higher rotational speeds, resulting in an incorrectly slow speedometer reading when the vehicle is traveling fast.

One explanation that makes sense under the circumstances is that there is something mis-matched between your drive ratios and speedometer. If the speedometer is calibrated to give correct readings with an overdrive unit that is not present I can envision it behaving as it does. At lower speeds, before the overdrive would normally engage, it reads correctly, but at higher speeds, when the overdrive would engage if it were present, the speedometer thinks the truck is going much faster than it really is.