80 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:

80 × 1.609344 = 128.7 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 80 mph a compliant dashboard may legally display anything from 80 mph up to roughly 90.5 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 80 mph?

80 mph equals 128.7 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 80 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 80 mph a compliant dashboard may legally display up to about 90.5 mph.