Updated September 2026
The formula
Cost = (distance ÷ MPG) × price per gallon
In metric: (distance ÷ 100) × L/100 km × price per litre.
Worked through: 450 miles, a car that averages 28 MPG, fuel at $3.29 a gallon.
- Fuel needed: 450 ÷ 28 = 16.07 gallons
- Cost: 16.07 × $3.29 = $52.88
- Per mile: 11.8 cents
- Four people in the car: $13.22 each
Round trip doubles it, which sounds obvious until you are quoting someone the one-way number and wondering why the weekend cost more than you said it would.
Which MPG figure to use
| Trip | Use | Then adjust |
|---|---|---|
| Motorway road trip, light load | EPA highway | Nothing |
| Motorway, four people and luggage | EPA highway | Take off 10% |
| Motorway with a roof box or bikes | EPA highway | Take off 15 to 25% |
| Mixed roads | EPA combined | Nothing |
| City only | EPA city | Take off 10% in winter |
| Mountains, towing, roof rack | EPA city | Take off another 15% |
All of that is guesswork sitting on top of a manufacturer's test figure. Your own logged average beats every row in that table, and it takes two fill-ups to start having one.
What common trips cost
Fuel only, one way, at $3.29 a gallon. Find the row nearest your car and scale it.
| Distance | 20 MPG (SUV) | 28 MPG (sedan) | 40 MPG (hybrid) |
|---|---|---|---|
| 100 mi | $16.45 | $11.75 | $8.23 |
| 250 mi | $41.13 | $29.38 | $20.56 |
| 500 mi | $82.25 | $58.75 | $41.13 |
| 1,000 mi | $164.50 | $117.50 | $82.25 |
| 2,800 mi (coast to coast) | $460.60 | $329.00 | $230.30 |
The gap is the whole argument for knowing your own number. Across the country, the SUV costs $230 more in fuel than the hybrid, and that is before anyone has eaten.
Four things that actually reduce the bill
Slow down
Drag scales with the square of speed. Dropping from 80 to 70 mph on a long motorway run typically saves 10 to 15 percent of the fuel, and on a 500 mile day costs you about half an hour.
Take the roof box off
An empty roof box costs fuel every mile it is up there. Several MPG at motorway speed is normal. If it is not carrying anything this weekend, take it off.
Check the tyres before you leave
Under-inflated tyres are worth a few percent, and everyone's tyres are under-inflated in the first cold snap of the year. It costs nothing and takes five minutes at the pump.
Fill up before the last exit
Motorway service station fuel runs 30 to 60 cents a gallon above a town station a mile off the exit. On a full tank that is $5 to $9, for a two minute detour.