Calibrating the speedometer

TL;DR how to calibrate the speedo

The gauges are built by Caerbont Automotive Instruments , they are the current owner of the SMITHS brand and manufature gauges with various brand names. They are the ones supplying Caterham.

As they can be fitted to about any car, you have to set them up accordingly, by setting a "number of pulses per mile". Those are digital units with a stepper motor.

The user guide is available on the Caerbont website and here is how to adjust the PPU :

1. With the ignition off, keep the trip-reset button depressed 
2. Switch the ignition on keeping the trip-reset button depressed
3. Release the button and press it again until "SET PPU" is shown on the display
4. Press the button again maitaining it for 2-3 sec
5. Read the current calibration value : On my car it was set to 022364 and the car did show 130kph for a GPS speed of only 120kph. We will need to adjust it to 130 / 120 * 22364 = 024228
6. Adjust the calibration value one digit at a time by pressing the trip reset button, each press will increase the current digit and a pause will send the cursor to the next digit.
7. After reaching the final digit, the whole value will blink and you can confirm by pressing the button one final time
8. The screen will read "DONE" - you can turn the ignition off.

My speedometer is now spot on.