The formula
time = distance ÷ speed + stops
Where it comes from
The average speed over a long trip is always lower than the cruising speed: between the urban stretches at each end, the tolls and the queues, a motorway journey rarely averages more than 100 km/h even at a steady 120.
How to work it out by hand
- Divide the distance by the expected average speed
- Add the minutes of every stop
- Convert the total to hours and minutes
What is worth knowing
Two fifteen-minute stops on a six-hour trip pull the real average from 100 down to 90 km/h. And here is a figure that surprises people: going from 110 to 130 km/h over 500 km saves about 42 minutes but raises consumption by around 25 %. The advice to rest every two hours exists for a reason, and the time it costs is less than it looks.