How to use the Fuel Cost
- Enter the distance — one way or return, being explicit about which.
- Enter the vehicle's efficiency in MPG or litres per 100 km.
- Enter the current fuel price per gallon or litre.
- For an annual figure, enter your yearly mileage instead of a trip distance.
How the calculation works
With MPG, fuel used is distance ÷ MPG and cost is that times the price per gallon. With litres per 100 km, fuel used is distance ÷ 100 × L/100km. The two measures are reciprocal, which produces a counter-intuitive result: improving from 15 to 20 MPG saves more fuel over the same distance than improving from 30 to 40, because the saving depends on the change in fuel per mile rather than the change in miles per gallon.
Real-world consumption usually falls short of the official figure. Motorway speeds above about 110 km/h increase aerodynamic drag sharply, short urban journeys run the engine cold and inefficient, roof boxes and heavy loads add drag and mass, and under-inflated tyres raise rolling resistance. A 10 to 20% margin over the manufacturer's figure is a realistic planning assumption for mixed driving.
Cost = (distance / MPG) × price per gallon ; metric: (distance / 100 × L/100km) × price per litreSource: Standard fuel consumption arithmetic; real-world variance from official test figures per EPA and WLTP published guidance.
Worked example
A 540-mile return trip in a vehicle averaging 34 MPG, fuel at 3.62 per gallon, compared with a 26 MPG vehicle.
- 34 MPG: 540 / 34 = 15.88 gallons × 3.62 = 57.49.
- 26 MPG: 540 / 26 = 20.77 gallons × 3.62 = 75.19.
- Difference on this trip = 17.70.
About 57 versus 75 for the same journey — around 640 a year apart at 20,000 miles of similar driving.
Frequently asked questions
Why is my real MPG worse than the official figure?+
Official figures come from standardised tests. High motorway speeds, short cold journeys, roof loads, air conditioning and under-inflated tyres all push real consumption 10 to 20% higher.
Does driving slower really save fuel?+
Above roughly 110 km/h, aerodynamic drag rises steeply, so reducing motorway speed noticeably cuts consumption. In town, smooth acceleration and anticipation matter more than speed.
Why does upgrading a thirsty car save more than upgrading an efficient one?+
Because fuel used per mile is the reciprocal of MPG. Going from 15 to 20 MPG saves more fuel over the same distance than going from 30 to 40, despite the smaller MPG gain.
How do I compare with an electric vehicle?+
Convert both to cost per mile: fuel price ÷ MPG for petrol, and electricity price × kWh per mile for an EV. That puts very different units on a comparable basis.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.