How this is worked out
Fuel cost is three numbers multiplied in the right order:
gallons = distance ÷ MPG
cost = gallons × price per gallon
300 miles at 30 MPG is exactly 10 US gallons, and at $3.20 a gallon that is $32.00 — about 10.7 cents a mile. A round trip doubles it. Four people sharing makes it $8.00 each.
Internally this page works in litres per kilometre, because that is the only way to hold MPG, L/100km and km/L in the same calculation without a rounding error creeping in. The conversion is exact rather than approximate: a US gallon is 3.785411784 litres by definition, an imperial gallon is 4.54609 litres, and a mile is 1.609344 kilometres.
Those definitions are the reason for the four-unit table under the result. 30 MPG (US) and 30 MPG (imperial) are different cars. The imperial gallon is about 20% larger, so 30 MPG (US) is 36.0 MPG (imperial), and a British review quoting 30 MPG is describing a car that would be rated 25.0 MPG in the United States.
A worked example
- Trip distance
- 300 miles, one way
- Fuel economy
- 30 MPG (US)
- Gas price
- $3.20 per US gallon — a placeholder, not a quote
- Fuel used
- 10.00 US gallons — 37.85 litres
- Trip cost
- $32.00
- Cost per mile
- $0.11 — $10.67 per 100 miles
- Split four ways
- $8.00 each
- 12,000 miles a year
- $1,280.00 — $106.67 a month
- The same car at 40 MPG
- $960.00 a year — $320.00 saved
- 30 MPG (US) is also
- 36.0 MPG (imperial) · 7.84 L/100km · 12.75 km/L
A US gallon is not an imperial gallon, and the gap is 20%
The US gallon is 231 cubic inches, which is 3.785411784 litres. The imperial gallon is 4.54609 litres. Both are exact by definition, and the imperial one is 20.1% larger — a difference that has survived since the British redefined their gallon in 1824, decades after the United States had settled on the older wine gallon.
The consequence is that MPG is not one unit but two, and nothing on either number says which. 30 MPG (US) is 36.0 MPG (imperial). A car reviewed in Britain at 45 MPG would be rated 37.5 MPG on an American window sticker. Reading a foreign road test without converting produces an error of a fifth, always in the flattering direction for the imperial figure.
That is why this page shows every economy figure in all four units at once rather than making you choose an output. Enter what you have, read whichever line matches the number you are comparing against.
MPG is a reciprocal scale, so improvements do not compare
MPG measures distance per unit of fuel, so the fuel burned over a fixed distance is proportional to 1 ÷ MPG. That makes equal steps on the scale wildly unequal in fuel. Over 12,000 miles, going from 20 to 25 MPG saves 120 gallons. Going from 40 to 50 MPG — the same five MPG, the same 25% improvement — saves 60 gallons, exactly half as much.
The effect is sharper at the bottom. Replacing a 15 MPG truck with a 20 MPG one saves 200 gallons over the same 12,000 miles — exactly as much as replacing a 30 MPG sedan with a 60 MPG hybrid, which sounds like a far bigger change and is not. This is the "MPG illusion", and it is why fuel economy labels in the US now carry gallons per 100 miles beside the MPG figure.
The practical rule: compare fuel used, not MPG. Enter both cars on this page and read the annual cost, because dollars are a scale where equal differences mean equal things and MPG is not.
The window sticker, and what you actually get
EPA ratings come from standardised laboratory cycles with an adjustment applied, and they are a fair basis for comparing two cars. They are not a promise about your commute. Short trips where the engine never warms up, sustained speeds above about 60 mph, roof boxes, underinflated tires and cold weather all cost real fuel.
Winter is the underestimated one. Cold engines run rich, batteries and cabin heaters draw more, winter-blend gasoline carries slightly less energy per gallon, and hybrids and EVs lose more than conventional cars because their efficiency advantages depend on a warm system. A 15–25% penalty on short winter trips is ordinary.
The honest way to use this page is to fill your tank, reset the trip meter, refill, and divide miles by gallons. That number is your MPG, and it will usually be a few points below the sticker.
Fuel is not the cost of driving a mile
Ten or twelve cents a mile in gas feels cheap, and it is — because it is a small part of what a mile costs. Depreciation is usually the largest single component of car ownership, followed by insurance, then maintenance and tires, then fuel. The IRS standard mileage rate for business driving exists precisely because the full cost is several times the fuel-only figure.
That matters when the calculation is being used to decide something. Fuel cost alone is the right number for "should we take one car or two", or "is the cheaper gas station across town worth the detour". It is the wrong number for "should I drive or fly", and badly wrong for "should I buy a more efficient car", where the purchase price, insurance and depreciation usually dominate the fuel saving.
The comparison panel on this page is fuel only and says so. Use it as one input to that decision, and price the rest separately rather than assuming the fuel saving carries the whole case.
Assumptions and sources
- US gallon
- 231 cubic inches = 3.785411784 litres, exact by definition. NIST Handbook 44. https://www.nist.gov/pml/owm
- Imperial gallon
- 4.54609 litres, exact by definition since 1985. https://www.legislation.gov.uk/ukpga/1985/72
- Mile
- 1.609344 kilometres exactly, from the international yard and pound agreement of 1959.
- Fuel price
- The default on this page is a PLACEHOLDER, not a researched figure. Prices move weekly and differ by more than a dollar a gallon between states — use the price you paid.
- Fuel economy ratings
- EPA combined figures from standardised test cycles. https://www.fueleconomy.gov/
- Arithmetic
- tripCost, annualFuel and efficiencyIn in assets/js/tools/work.js, verified in tools/test/work.mjs against hand-computed gallons.