Use your real economy, not the sticker
The answer is only as good as the economy figure you type in. The window sticker is a laboratory result, and few drivers match it: cold weather, short trips, a roof box, a full car, and highway speeds above 70 all push real consumption higher.
Read the figure off your trip computer over a few tanks, or measure it properly with the gas mileage calculator and bring that number back here.
The gap between the sticker and your own figure is mostly habit and load, and the Department of Energy puts a number on each cause.
| Cause | Effect on fuel economy |
|---|---|
| Aggressive acceleration and braking | 10 to 40 percent worse in stop-and-go traffic |
| Every 5 mph above 50 mph | Roughly like paying an extra 20 to 30 cents a gallon |
| Roof-mounted cargo box | 6 to 17 percent worse at highway speed |
| Short trips at 20F against 77F | About 15 percent worse while the engine warms |
Run the calculator twice, once at your summer figure and once at your winter one. The difference is the seasonal cost you have been absorbing without noticing. A 10 percent optimistic economy figure understates a year of commuting by roughly a month of fuel.
The commute line is the one that matters
A single trip cost is easy to absorb. The same trip every week is a standing cost, so the calculator converts it for you at 52 weeks over 12 months.
That is the figure to carry into a buying decision. A car that returns 10 mpg more on the same commute saves a meaningful share of it every year, which is why fuel economy belongs in the affordability calculation rather than in the nice-to-have column.
What the calculator does and does not cover
Switch between miles, gallons and mpg, and kilometres, litres and litres per 100 km, using the toggle. The switch changes what you type and read, never the arithmetic.
This prices fuel and nothing else. Tolls, parking, tyres, and the wear that mileage puts on a car are real trip costs that sit outside it.
It also assumes petrol or diesel. For an electric car the equivalent calculation runs on kilowatt hours and your electricity tariff instead, which is covered in EV charging basics.