calibrate speedometer

[analog_me]

Hi to all,

My speedometer must be miles off and would like to calibrate this. do you know who can do this please???
It is a 99 car so a caterham cable operated instrument.

In comparison to the GPS speed is not so much off (anyway the needle is bouncing a little bit so hard to tell exactly your speed) but when comparing the distance traveled this is over reading by 21%
That is judge!

The car was retrofitted a 6 speed with a 3.92 diff and is on 175x60R13

Thanks