Why your speedometer reads a little high — in any country

The exact factor

One mile per hour is exactly 1.609344 kilometres per hour, following directly from the international mile of 1,609.344 metres agreed in 1959. Unlike some of the shortcuts on this site, there is no simpler "close enough" version worth memorising for speed — the rough multiply-by-1.6 rule most people use is accurate to within about half a per cent, which is fine for a mental estimate and not what your car's electronics use.

Why the dashboard number is not quite the true number

Regulations in most countries — UK type-approval rules and equivalents elsewhere — require a speedometer to never display a speed lower than the vehicle's actual speed, with a permitted over-read of a few per cent. Manufacturers calibrate to the safe side of that margin rather than to zero error, so a speedometer reading 70 mph is often doing genuine 66–68 mph. This is a deliberate legal safety margin, not a fault, and it exists independently of any unit conversion.

The two effects compound in a rental car abroad. A UK driver renting in the US sees km/h nowhere — US speedometers are mph-primary — but the reverse trip, a US driver in Europe, hits a genuinely dual-scale dial, and misreading the smaller of the two rings by habit is a well-documented rental mistake.

Reference table for common limits

Cruise control and GPS tell a more honest story

Modern cruise control and adaptive cruise systems typically reference the same wheel-speed sensors as the speedometer, so they inherit the same small over-read — set to an indicated 70 mph and you are usually travelling a little under that. GPS-based speed readouts, by contrast, measure ground speed directly from satellite positioning and are not subject to the same regulatory margin, which is why a phone's GPS speed and the car's dashboard rarely agree exactly, and why the GPS figure is generally the more accurate of the two.

Tyre size changes the number again

Because the speedometer is calibrated against wheel rotations and a standard rolling circumference, fitting a tyre with a different overall diameter shifts the reading independently of the factory margin — a point covered in more detail in reading a tyre sidewall. The two effects — legal over-read and tyre-diameter drift — stack, so a modified vehicle's dashboard can be a genuinely unreliable speed reference.

The takeaway

Multiply mph by 1.609344 for an exact km/h figure, and remember the number on the dial was never meant to be exact in the first place — it is built to round in the safe direction.