How the trip cost is calculated
Three numbers decide what a drive costs: how far you go, how far your car travels on a gallon, and what a gallon costs. Distance divided by fuel economy tells you how many gallons the trip will burn, and multiplying those gallons by the pump price gives the total.
The cost per mile is the same total spread evenly across the distance. It is the most useful figure for comparing trips or vehicles, because it strips out trip length and leaves a clean rate you can multiply by any future mileage.
Making the estimate realistic
The single biggest source of error is the fuel-economy figure. A car rarely hits one fixed number; it does better cruising on the highway and worse in stop-and-go traffic, cold weather or with a heavy load.
- Use your real average from recent fill-ups rather than the window-sticker rating.
- For mixed driving, a combined economy figure beats picking pure city or pure highway.
- Remember terrain, air conditioning, roof racks and aggressive acceleration all push economy down.
Reading the cost curve
The chart traces the running fuel cost as the trip progresses, climbing in a straight line because each mile costs the same. It makes the cost-per-mile rate visual: the steeper the line, the more each mile is costing you, whether from a low-economy vehicle or a high fuel price.
What it does not include
This figure is fuel only. The true cost of driving also includes wear on tires and brakes, maintenance, depreciation, tolls and parking. For a complete cost-of-ownership picture, treat fuel as one line item among several rather than the whole bill.
Formula
cost = (distance / mpg) × priceFrequently asked questions
- Should I use city or highway MPG?
- Use whichever matches your trip. For mixed driving, a combined MPG figure gives the most realistic estimate.