The formula
Distance = speed × time. Multiplying the average speed you expect to maintain by the number of hours available gives the distance you can cover — the reverse operation from calculating driving time, which divides instead of multiplies.
A worked example
Driving at a steady average of 60 km/h for 4.5 hours covers 60 × 4.5 = 270 km. At a faster average of 100 km/h for 2.5 hours, the distance covered is 100 × 2.5 = 250 km — a shorter time window at a higher average speed can cover nearly as much ground as a longer window at a lower one.
When this direction is the useful one
This calculation answers a different everyday question than driving-time calculations do — instead of "how long will this trip take," it answers "how far can I get" for a fixed time budget, such as planning how far a day's drive can reasonably reach before needing to stop for the night, or checking whether a destination is reachable within the time before an appointment.
Converting a fractional-hour time budget correctly
A time budget expressed as "2 hours 30 minutes" needs converting to decimal hours before multiplying — 30 minutes is 0.5 hours, so 2 hours 30 minutes becomes 2.5 hours. Skipping this conversion and using "2.30" instead of "2.5" is a common data-entry mistake that would understate the distance calculation.
Why this is an upper-bound estimate, not a guarantee
Like the driving-time direction of this same formula, this calculation assumes the entire time budget is spent driving at the stated average speed, with no stops or slowdowns factored in — it's best treated as the maximum distance achievable under those conditions, with a buffer subtracted for anything that would reduce the actual average speed along the way.