Speedometers

A speedometer shows road speed, and after a gearbox swap, a final drive change or new tyre sizes, the factory one can read wrong. The speedometer in this section is a black gauge reading to 250 km/h, listed at 100 × 100 mm with a height of 65 mm.

A stand-alone electronic speedometer counts pulses from a speed sensor or from a speed signal the car already has. It is calibrated by telling it how many pulses equal a set distance, which takes care of tyre size and gearing. A mechanical speedometer uses a drive cable from the gearbox instead, so check which input the listing describes.

For road use, err on the high side. A speedometer that shows less than the true speed puts the driver at risk of speeding without knowing it, so calibrate over a measured distance or against GPS.

Points to check:

• Scale: up to 250 km/h, which covers most track speeds while keeping road speeds readable.
• Size: listed at 100 × 100 mm, which needs a larger opening than a small gauge pod offers; check the space behind the panel too.
• Signal source: a gearbox or hub sensor, or a speed output from the ECU or ABS module. Where the car has none, a separate speed sensor can supply one.

Mount the sensor with the gap its maker specifies from the teeth or magnets it reads, and secure its cable away from moving parts and heat. Shield the signal wire from ignition noise, which shows as a jumping needle. Recalibrate after any change of tyre size or final drive.