mile per hour

Miles per hour is the speed unit of two countries and a surprising number of habits elsewhere. It is exactly 0.44704 m/s, which follows from the mile being exactly 1,609.344 m and the hour being exactly 3,600 s, so the conversion involves no measurement and no rounding. The neatest identity in the unit is that 60 mph is exactly 88 ft/s, since 60 miles is 316,800 feet and an hour is 3,600 seconds. Stopping distance calculations in American driver training lean on this: at 88 ft/s, a one-second reaction time is 88 ft of road gone before the brakes touch.

Two conversions are worth carrying. To go from mph to km/h, multiply by 1.609, so 70 mph is 113 km/h and 30 mph is 48 km/h. To go from mph to m/s, divide by 2.237, or just halve and take off a bit: 60 mph is about 27 m/s. And because every force that depends on speed through the air scales with speed squared, the drag at 80 mph is not a third more than at 60 mph but nearly double, which is why fuel economy falls off a cliff above motorway speeds.