Seventy miles per hour is 112.654 km/h and 100 km/h is 62.137 mph, so no speed limit converts to a round number in the other system. Both countries chose limits that were tidy in their own unit, which is why European motorways run at 110, 120 and 130 km/h rather than at the 112.7 and 128.7 that would match 70 and 80 mph.
The practical consequence for a driver abroad is that converting the sign is the easy half. The harder half is that the needle in front of you is not required to be accurate.
What do the common limits convert to?
The mile is exactly 1,609.344 metres, so every figure here is a definition rather than a measurement.
| mph | km/h | km/h | mph | |
|---|---|---|---|---|
| 20 | 32.187 | 30 | 18.641 | |
| 30 | 48.280 | 50 | 31.069 | |
| 40 | 64.374 | 80 | 49.710 | |
| 55 | 88.514 | 100 | 62.137 | |
| 65 | 104.607 | 110 | 68.351 | |
| 70 | 112.654 | 130 | 80.778 |
The near-pairs are worth noticing because they are the ones that catch people. A 30 mph urban limit is 48.3 km/h, close enough to a 50 km/h sign to feel identical and 1.7 km/h more permissive. A 130 km/h autobahn section is 80.8 mph.
Why is the speedometer allowed to be wrong?
Because the regulations only forbid it from being wrong in one direction. Under the UN vehicle regulation applied across Europe and the UK, an indicated speed may never be lower than the true speed, and may exceed it by up to 10 per cent plus 4 km/h. So at a true 100 km/h a legal speedometer may read anything from 100 to 114 km/h.
That is why a satellite-derived speed reads consistently lower than the dashboard. The satellite figure is a measurement; the dashboard figure is a measurement plus a deliberate safety margin, so that no manufacturer ever ships a car that under-reads.
Odometers are held to a different and tighter standard, which is why distance travelled matches the map while speed does not.
Does the conversion trick work?
Adding 60 per cent converts miles to kilometres to better than one per cent all the way up: 60 miles plus 60 per cent is 96, against a true 96.56. Going the other way, take a third off and add a bit — 100 km/h less a third is 66.7, against a true 62.1, so that direction is worse and worth doing properly.
The genuinely reliable trick is the Fibonacci pairing, because the golden ratio of 1.618 sits within half a per cent of 1.609344: 50 and 80, 80 and 130. A 50 mph limit is about 80 km/h and a 130 km/h limit is about 80 mph, both correct to under a mile an hour.
What should you actually drive at?
The sign, read in the local unit, with the dashboard treated as an over-reading instrument. If the speedometer says 125 in a 120 zone, you are probably doing about 118, and if it says 120 you are certainly not doing more.
Cars sold across both markets usually show both scales, with the secondary one in smaller figures on the inner ring. Reading the inner ring is more reliable than any mental arithmetic, and it is the one thing worth locating before you leave the rental car park.
What about enforcement tolerance?
Enforcement tolerance and speedometer tolerance are separate things, and only one of them is in your favour. The speedometer margin means your true speed is at or below what the dial shows. Any enforcement allowance on top of that is a policy of the authority doing the measuring, varies by country, and is not something to plan a journey around.
The arithmetic that matters is the conversion of the limit, done once, before you set off. A 110 km/h limit is 68.35 mph, so a driver used to imperial figures should be thinking "just under 70" rather than converting at every sign.
How do you read a dual-scale dial?
The primary scale is the larger set of figures on the outer ring, and it matches the market the car was sold into. The secondary scale sits inside it in smaller type, and on many dials it is not evenly spaced against the outer one, because the two sets of round numbers do not line up.
Digital displays usually let you switch units outright, which is better than reading a converted ring. Do it in the car park rather than on a motorway slip road, and check the odometer at the same time — a car showing miles on the clock and kilometres on the speed display is a genuine and confusing configuration.
Questions people ask
Does the same factor convert speeds and distances? Yes. Miles per hour and miles both multiply by 1.609344, because the hour is the same in both systems.
Why is a mile 1,609.344 metres? It is 1,760 yards, and the yard was defined as exactly 0.9144 m in 1959. The awkward metre figure is what falls out of a tidy yard figure.
Are UK and US miles the same? For road purposes, yes — both use the international mile. The old US survey mile differed by about 3 mm in a mile and was withdrawn from use at the end of 2022.
What about nautical miles? A different unit entirely, at exactly 1,852 m, defined as one minute of latitude. Knots are nautical miles per hour and appear at sea and in the air, never on a road sign.
Convert the sign, then trust the sign over the needle. Miles to kilometres and kilometres to miles do the arithmetic, and litres to gallons covers the other unit you will meet at the first filling station.