Speedometer to slow

[Uwe Koch]

79 Z28 the speedometer shows more then 20percent less then correct speed. Changed the gear driven from red to brown, now it is faster, but still much to slow, minimum 10 km/h, the more the faster the car is going. It is all factory setup, turbine wheels in 15 with 225/70 tires on, no modification in trans or rear end. Couldn't imagine the cable is the cause, could there be any internal problem in the speedometer unit? No more idea on my side, thanks for any comments, Uwe

 

[camarolife78]

I might be wrong, but isn't the correct size tire like a 225/60R15? or 225/55R15? I run 255/55R15 on the back and my speedo is close, sounds like your tire might be too tall.

 

[Zspoiler]

The cable probably needs to be greased.

 

[cadillac_al]

How do you know it is original trans? Chances are it got replaced at some point and the speedo gears aren't correct. There are charts out there somewhere that show what speedo gears go with what axle ratio/tire size.

 

[70-camaro]

http://www.tciauto.com/tc/speedometer-gear-calculator/

 

[Dave Nelson]

Here's another calculator.
https://www.ringpinion.com/calculators/Calc_RPM.aspx