Average Speed Round Trip Calculator
Drive out at one speed and back at another over the same distance — your true average speed is the harmonic mean, not the simple average.
How it works
Average speed = 2·v₁·v₂ ÷ (v₁ + v₂) (harmonic mean of the two speeds)
For equal distances, the correct average is the harmonic mean, not the arithmetic mean, because you spend more time on the slower leg. Total distance divided by total time is what “average speed” means, and the extra time crawling back at the low speed drags the true average below the halfway point of the two numbers. The gap grows as the two speeds get further apart.
Worked example
Drive 60 miles out at 60 mi/hr (1 hour) and back at 40 mi/hr (1.5 hours). Total is 120 miles in 2.5 hours = 48 mi/hr. The formula agrees: 2·60·40 ÷ (60 + 40) = 4800 ÷ 100 = 48 — not the naive 50.
Frequently asked questions
Why isn’t the average just (60 + 40) ÷ 2 = 50?
Because you spend more time at the slower speed. Average speed is total distance ÷ total time, and the longer slow leg pulls the true average down to 48 mi/hr.
Does the distance of the trip matter?
No, as long as both legs cover the same distance. The distance cancels out of the formula, so a 5-mile or 500-mile round trip at the same two speeds gives the same average.
What if the two legs are different distances?
Then this harmonic-mean shortcut no longer applies. Compute each leg’s time separately and divide the total distance by the total time instead.
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This calculator is for educational purposes. Double-check important results before acting on them.