4 mph to km/h

The arithmetic

One international mile is exactly 1.609344 kilometres, so this is a single multiplication with nothing approximated except the final rounding:

4 × 1.609344 = 6.4 km/h

What your dashboard may show at this speed

A car speedometer is not allowed to read below your true speed, and it is allowed to read a good deal above it. Under UN Regulation No 39 the indicated speed may exceed true speed by up to 10% plus 4 km/h. At a true 4 mph a compliant dashboard may legally display anything from 4 mph up to roughly 6.8 mph, and most cars sit inside that band rather than at the top of it.

That gap is why a GPS reading and a dashboard reading rarely agree, and why the dashboard is the wrong instrument for judging how close you are to a limit. The regulation behind it is explained here.

Common questions

How many km/h is 4 mph?

4 mph equals 6.4 km/h. The conversion uses the exact factor of 1.609344 kilometres per mile.

Is the mph to km/h conversion exact?

The factor is exact by definition: one international mile is exactly 1.609344 kilometres. The figure here is that result rounded to one decimal place.

Why does my car show more than 4 mph?

Car speedometers may never read below true speed and may read up to 10% plus 4 km/h above it under UN Regulation No 39. At a true 4 mph a compliant dashboard may legally display up to about 6.8 mph.