Speedo calibration

[J5]

The bad thing about this is the additional 10% of accumulated miles that are being tacked on to your odometer. 

nope

rules often state that speedo can under read by 10% , 0& over read and odometer must read actual

if you fix the speedo error you then get 10% less miles on the odometer

[Speeddog]

For you guys with the mechanical/cable speedos.

The odometer reading is a direct-drive system from the speedo cable.
Likely it's pretty accurate.

The speedometer needle is (usually) hooked to a shallow steel cup, with a light return spring.
The cable hooks to a magnet disc situated inside the steel cup, that it spins.
So the speedo reading is a function of how far the spinning magnetic disc can turn the cup against the return spring.

Innaccurate speedo does not automatically mean inaccurate odometer.