Average Calculator · Practical guide
Average Speed for a Round Trip
The average of two speeds is not the average speed — round trips over equal distances need the harmonic mean.
At a glance
The Short Answer
If you drive the same distance out and back, your average speed is the harmonic mean of the two speeds, not their simple average. Drive 60 mph out and 40 mph back and your round-trip average is 2 ÷ (1/60 + 1/40) = 48 mph — not 50 mph. Enter 60, 40 in the calculator above with Harmonic Mean selected to confirm it.
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Enter your own values, then compare the result with the examples below.
Result
Why the Simple Average Fails
You spend more time driving at the slower speed. On a 120-mile round trip (60 miles each way), the outbound leg takes 1 hour at 60 mph, but the return takes 1.5 hours at 40 mph. Total: 120 miles in 2.5 hours = 48 mph. The slow leg eats more clock time, so it drags the true average below the midpoint of the two speeds.
Round-Trip Averages for Common Speed Pairs
| Speed Out | Speed Back | Simple Average (Wrong) | True Average (Harmonic) |
|---|---|---|---|
| 30 mph | 60 mph | 45 mph | 40 mph |
| 40 mph | 60 mph | 50 mph | 48 mph |
| 50 mph | 70 mph | 60 mph | 58.33 mph |
| 60 mph | 60 mph | 60 mph | 60 mph |
The bigger the gap between the two speeds, the further the true average falls below the simple average.
When the Simple Average Is Right
If you drive each speed for equal time instead of equal distance — one hour at 60 mph, one hour at 40 mph — the arithmetic mean is correct: 100 miles in 2 hours = 50 mph. The rule: equal distances call for the harmonic mean; equal times call for the arithmetic mean. The same harmonic-mean math applies anywhere you average rates over a fixed quantity, like fuel economy or parallel resistors in electronics.
Frequently Asked Questions
What is the formula for average speed on a round trip?
For speeds a and b over equal distances: average speed = 2ab ÷ (a + b). For 60 and 40 mph that gives 2 × 60 × 40 ÷ 100 = 48 mph.
Why is my round-trip average lower than the two speeds' midpoint?
Because you spend longer traveling at the slower speed. Time, not distance, is what the speeds divide, and the slow leg dominates the total time.
Does traffic or stops change the calculation?
Yes. Any time spent stopped is time at 0 mph on that leg, which lowers the average further. Use your actual door-to-door times for a realistic figure.
Can I average three or more legs?
Yes. For equal-distance legs, the harmonic mean generalizes to the count divided by the sum of the reciprocals — exactly what the harmonic option above computes for any list of speeds.