The formula
average = distance ÷ time · effective = distance ÷ (time + stops)
Where it comes from
If you drive half the way at 120 and half at 40, the average is not 80: it is 60. The reason is that you spend three times as long on the slow half, so it weighs three times as much. Averaging speeds only works when each one lasts the same, and on a journey that almost never happens. That is why the average is always distance divided by time, never a sum of speeds.
How to work it out by hand
- Turn the driving time into hours: the hours plus the minutes divided by sixty
- Divide the distance by that time for the driving average
- Add the minutes spent stopped to the time
- Divide the distance by the total time again: that is the trip average
What is worth knowing
The gap between the two figures is what the stops cost, and it usually surprises people: two fifteen-minute stops on a two-and-a-half-hour trip pull the effective average down by nearly thirteen kilometres per hour, more than you would gain by raising your cruising speed. That is why on a long journey you arrive sooner by stopping less often than by driving faster — and with less fuel used and less risk.