Speedometers

Mechanical and electronic speedometers, plus the drive gears, senders and adapters that make them read correctly — over 250 parts.

Speedometer error is the most common unintended consequence of a tire or gear change. Fitting taller tires makes the vehicle travel further per revolution, so an uncorrected speedometer under-reads — you are going faster than it says, and the odometer under-counts distance with it. A gear change does the same in the other direction.

The correction depends on the mechanism. A cable-driven speedometer is recalibrated by changing the drive gear at the transmission; an electronic one needs a calibration module or a tuner that can be told the new tire diameter and axle ratio. Establish which you have before buying — the two solutions do not substitute for each other.