Average-speed trap
Why you cannot average speeds.
Black: the trip, leg by leg. Red dashed: one straight line from start to finish — its slope is the average speed.
- 50
- (60 + 40) ÷ 2
- 46.7
- 140 km ÷ 3 h
They differ by 3.3 km/h. The slower leg (40 km/h) takes 2 of the 3 hours, so the true average sits below the mean of the speeds. Average speed is total distance over total time — always.