Use miles per minute and fractions. Also applying percentage differences will help if the numbers are awkward...
50 miles at 90 mph? That's 50 at 1 1/2 (or 3/2) miles per minute. So, what's 50 x 2/3?
kph to mph? Multiply by .7, and knock off a tenth. So 80 kph is 56 minus 5.6 or roughly 50 mph.