Fuel is the cost most people estimate worst, because it is paid in small amounts and thought about in miles per gallon rather than dollars per mile. Converting to cost per mile makes comparisons immediate: it tells you whether a longer route on cheaper fuel is worth it, what a commute really costs over a year, and whether a more efficient car pays for itself.
The calculation is distance ÷ efficiency × price. The trap is unit consistency — mixing US and imperial gallons produces an error of about 20%, and a US gallon is the smaller of the two.
Efficiency measured three ways
- Miles per gallon (US): standard in the United States, based on the 3.785-litre gallon.
- Miles per gallon (imperial): used in the UK, based on the 4.546-litre gallon, so UK figures look about 20% better for the same car.
- Litres per 100 km: the metric convention, and the only one where a lower number is better.
- To convert: US mpg × 235.2 = L/100 km, and the same formula runs in reverse.
Annual fuel cost at 12,000 miles
| Efficiency | At $3.00/gal | At $4.00/gal | At $5.00/gal |
|---|---|---|---|
| 15 mpg | $2,400 | $3,200 | $4,000 |
| 25 mpg | $1,440 | $1,920 | $2,400 |
| 35 mpg | $1,029 | $1,371 | $1,714 |
| 50 mpg | $720 | $960 | $1,200 |
What actually changes your consumption
- Speed: consumption rises sharply above roughly 60 mph. Driving at 75 instead of 65 can cost 15–20% more fuel.
- Acceleration: hard starts and late braking are the largest controllable factor in town.
- Tyre pressure: under-inflation of 10 psi costs around 3%, and is free to fix.
- Weight and drag: a roof box can cost 10–25% at highway speed even when empty.
- Air conditioning: a few percent at speed, considerably more in stop-start traffic.
- Short trips: a cold engine can use twice the fuel for the first couple of miles.
Deciding whether cheaper fuel is worth the detour
Work out the saving on a full tank, then the fuel and time cost of getting there. Saving 20 cents a gallon on a 14-gallon fill is $2.80. A six-mile round trip in a 25 mpg car burns about a third of a gallon — roughly a dollar — leaving under two dollars of benefit for fifteen minutes of driving. For most people the detour is not worth it; for a van with a 30-gallon tank, it often is.
Related tools
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