1. The drive: transmission to cable

Road speed is read at the gearbox, not the wheels. A drive gear on the transmission (or transfer-case) output turns a matching driven gear, and that spins a squared-off inner cable running inside a flexible outer housing up to the back of the gauge. Because the pickup sits downstream of the gearbox, the reading is independent of which gear you're in — it tracks true road speed.

The ratio between drive and driven gear is what makes the speedometer read correctly for a given tyre size and axle ratio. Change your tyres, rear end or transmission and that ratio — the calibration — usually has to change with it.

2. The heart: a spinning magnet

At the gauge, the cable turns a small permanent magnet at the exact speed the cable spins. The magnet rotates freely, always in the same direction, inside a shallow non-magnetic aluminium cup.

3. The trick: eddy currents and the speed cup

As the magnet spins it drags its magnetic field across the aluminium speed cup. That moving field induces little swirling electrical currents — eddy currents — in the cup, which in turn create their own magnetic field. The cup is pulled in the direction the magnet is turning, trying to chase it. The magnet and cup never touch; only the field couples them. This is why the mechanism is nicknamed the eddy-current speedometer.

4. The balance: hairspring and needle

The speed cup carries the needle and is held back by a fine coiled hairspring. The faster the magnet spins, the harder it drags the cup; the further the cup turns, the harder the hairspring resists. The needle stops where those two forces balance. Double the cable speed and you roughly double the drag — so the scale reads smoothly and proportionally across its sweep.

5. The tally: the odometer

The same shaft drives a worm gear that steps down through a gear train to rotate the numbered odometer drums. Each drum clicks its neighbour over one place after the right distance. A separate resettable train drives the trip odometer. The heavy gear reduction is deliberate — fewer, slower-moving parts mean better accuracy and less to wear out.

Why it drifts over 50+ years

Nothing here is electronic, so nothing "fails" in the modern sense — it wears and gums up. The factory grease on the magnet spindle and odometer gears turns waxy and stiff. Pivots wear. The hairspring fatigues. Cables dry out and fray. The result is the familiar catalogue of classic-car complaints: a bouncing needle, a squealing cable, a lazy or sticking pointer, a frozen odometer. Every one traces back to friction or wear in this simple machine — and every one is repairable.