Plan fuel stops for a road trip.
Planning stops is a matter of usable range, not full range. Reserving 15 percent of the tank keeps you off the fuel light and out of trouble where stations are sparse, so the range that counts is capacity times economy times what is left after the reserve. You start with a full tank, so the number of stops is one fewer than the number of legs — dividing distance by range and rounding up gives legs, and subtracting one gives stops. Spacing them evenly is easier on the schedule than running each tank to the reserve.
Usable range
Usable range = tank capacity x mpg x (1 - reserve)
Stops
Stops = ceiling(total miles / usable range) - 1, floored at zero
Because the warning reserve is small — often 1.5 to 2.5 gallons — and station spacing on rural interstates can exceed that at motorway speeds. A planned 15 percent reserve costs nothing and removes the risk.
Use your steady-state motorway figure, which is often better than your mixed-driving average for a long trip but worse if the route is mountainous or you are loaded and towing.