Miles per Hour to Kilometers per Hour

The United States and the United Kingdom are the big users of miles per hour, joined by a few smaller countries and territories. In the UK the road signs stayed in mph when most other measures went metric, so the 70 mph motorway limit is 112.7 km/h, the 60 mph single-carriageway limit 96.6 km/h, the 30 mph built-up limit 48.3 km/h and the spreading 20 mph zones 32.2 km/h. US interstates typically range from 65 to 80 mph, or 105 to 129 km/h, and the old 55 mph national limit was 88.5 km/h.

Visitors from metric countries driving in the US or UK need the conversion the other way round from most travelers, and car hire companies there rarely fit km/h speedometers.

Car magazines in the US quote 0–60 mph times, while European ones quote 0–100 km/h. They are not the same test: 100 km/h is 62.1 mph, so a 0–100 time is a little longer than the 0–60 time for the same car. Sports speeds travel the same way: a 100 mph fastball is 160.9 km/h, and a tennis serve at 130 mph is 209 km/h.

Weather reports in the US give wind in mph. A Category 1 hurricane starts at sustained winds of 74 mph, which is 119 km/h, and a 40 mph gust is 64 km/h.

To estimate in your head, add 60%: 50 mph plus 30 is 80 km/h, against the exact 80.5. The Fibonacci pairs 5 and 8, 8 and 13, 13 and 21 also work because their ratio is close to 1.6. The common mistake is dividing instead of multiplying, which gives a smaller number; km/h figures are always larger than mph.

For the reverse direction see km/h to mph. Sailing and flying speeds use knots to mph.