Outward at 60, return at 40: why the round-trip average is not 50
When the distances are equal, the slower leg takes more time. Average speed must use total distance divided by total time.
Speed, distance and time calculator →Calculate each leg’s duration
An outward distance of 120 km at 60 km/h takes two hours. Returning the same 120 km at 40 km/h takes three hours. The total is 240 km in five hours, so average speed is 48 km/h rather than 50. Entering the combined distance and time verifies the result directly.
When a simple average does work
If the two speeds are maintained for equal times, such as one hour each, the distances are sixty and forty kilometres. The total becomes 100 km in two hours, giving 50 km/h. Equal-distance and equal-time conditions are different, and the correct weighting changes accordingly. An unweighted average should not be used until its assumptions have been checked.
Include actual stops when appropriate
If the round trip also contains one hour of rest, departure-to-arrival time is six hours and the whole-trip average becomes 40 km/h. The calculator does not know about that stop unless the time basis includes it. This analysis explains how averages behave; it is not advice to maintain a particular driving speed despite conditions. Keep the leg distances, times and stops separately so both moving and elapsed averages can be reproduced.
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