Speed = distance ÷ time, distance = speed × time, and time = distance ÷ speed. Pick which value to find; the field for that value is ignored and the other two are used. Time is entered as hours, minutes and seconds.
Example: driving 200 km at 80 km/h takes 200 ÷ 80 = 2.5 hours, which is 2 hours 30 minutes.
Divide the total distance by the total time. If you drove 150 miles in 2 hours 30 minutes, the average speed is 150 ÷ 2.5 = 60 mph.
Divide the distance by your average speed. 300 miles at 60 mph is 5 hours. Add stops and traffic separately, since they are not part of your moving speed.
Divide by 1.609. 100 km/h is about 62.1 mph. Use the km/h to mph converter.
See also: Pace Calculator · Km/h to mph · Fuel Cost Calculator