Average speed:
total distance over total time, and the trap in round trips

Total distance divided by total time

Cover 150 km in 3 hours and your average speed is 150 ÷ 3 = 50 km/h. Nothing in that sum cares whether you drove at 50 the whole way or crawled through a town and then sped on a highway. Average speed is a single figure that would get you the same distance in the same time at a constant pace.

Rearranged, the same relationship gives time = distance ÷ speed, and distance = speed × time. At 50 km/h, a 275 km journey takes 275 ÷ 50 = 5.5 hours, or 5 hours 30 minutes.

Stops, traffic and the clock

There are two ways to measure time, and they answer different questions. Moving time counts only the minutes the vehicle was rolling. Elapsed time includes every halt: fuel, tea breaks, toll queues, level crossings. For planning when you will arrive, elapsed time is what matters.

Take the 150 km trip again. Suppose you actually drove for 2.5 hours and took a 30-minute stop. Your moving average was 150 ÷ 2.5 = 60 km/h, but the journey average is 150 ÷ 3 = 50 km/h. Both are correct; they describe different things. A satnav showing a high moving average and a late arrival is not malfunctioning.

The round-trip trap

A frequent puzzle: you drive to a town at 60 km/h and come back at 40 km/h. What is the average? Adding and halving gives 50, which feels natural and is wrong. You spend more time at the slower speed, so it weighs more heavily.

Take a one-way distance of 120 km. Going takes 120 ÷ 60 = 2 hours. Returning takes 120 ÷ 40 = 3 hours. Total distance is 240 km, total time is 5 hours, so the average is 240 ÷ 5 = 48 km/h.

Check: 2 × 60 × 40 ÷ 100 = 48. Note that the one-way distance cancels out, so you get 48 km/h whether the town is 20 km or 500 km away. If the legs take equal time instead of equal distance, the simple average of 50 is correct.

Switching between km/h, m/s and mph

Athletes and physics problems tend to use m/s, vehicles use km/h, and some road signs abroad use mph. A 50 km/h average is 13.89 m/s, or about 31 mph. Convert before mixing figures from different sources, otherwise you can end up 'confirming' a speed that is off by a factor of 3.6.

Journeys made of several legs

For three or more legs, do not average the speeds. Add up the distances, add up the times, and divide once. Consider a trip with three legs: 40 km at 80 km/h, 60 km at 40 km/h and 30 km at 60 km/h.

• Leg 1 time: 40 ÷ 80 = 0.5 h
• Leg 2 time: 60 ÷ 40 = 1.5 h
• Leg 3 time: 30 ÷ 60 = 0.5 h
• Total distance 130 km, total time 2.5 h, average 52 km/h

A simple average of 80, 40 and 60 would give 60 km/h, which overstates the real figure by 8 km/h because the slow middle leg took the most time.

Putting the figure to work on a trip plan

Average speed is most useful as a planning input. Say you intend to drive 420 km and leave at 6:00 in the morning. On similar highways, with fuel and meal stops included, you have averaged about 52 km/h door to door. The journey would take 420 ÷ 52 = 8.08 hours, around 8 hours 5 minutes, putting you at the destination close to 2:05 in the afternoon.

Run the same sum with a more cautious 45 km/h and the drive becomes 9 hours 20 minutes, arriving about 3:20 pm. The gap between those two answers is more than an hour, which is why choosing a realistic average matters more than the precision of the division. If you are travelling on a road you have not used before, take the lower figure.

Runners and cyclists often think in pace rather than speed. Pace is minutes per kilometre, and speed is 60 divided by pace. A 5:00 min/km pace is 12 km/h, and 4:30 min/km is 13.33 km/h. A 10 km run at 12 km/h takes 10 ÷ 12 hours, which is 50 minutes.

What an average cannot tell you

Velocity is a related word that people confuse with speed. Average velocity is displacement divided by time, and displacement is the straight-line change in position. A round trip returns you to the start, so the displacement is zero and the average velocity is zero, even though the average speed is 48 km/h in the example above. Everyday travel questions almost always want speed.

An average hides variation. Two drivers can both log 50 km/h over 150 km, one at a steady pace and the other alternating between 20 and 80, and only the second needs hard braking and higher fuel use. For safety limits and speed enforcement, the instantaneous speed counts, not the average.

Use a speed average to estimate arrival times, compare routes or check a timetable. If you want to test several legs and stops quickly, an average speed calculator saves the repeated division, but the principle stays the same: total distance, total time, one division.