Fuel Cost Calculator
What a trip costs in gas
Planning a drive? Enter the distance, your vehicle's MPG and the gas price to see what the trip costs in fuel.
Trip fuel cost by economy
Used as a gas trip calculator or a broader trip cost calculator, the fuel line is the one that scales with distance.
The arithmetic is distance ÷ MPG × price per gallon. What that looks like on a 300-mile trip at $3.50 a gallon:
| MPG | Gallons | Trip cost | Cost per mile |
|---|---|---|---|
| 15 | 20.0 | $70.00 | $0.233 |
| 20 | 15.0 | $52.50 | $0.175 |
| 25 | 12.0 | $42.00 | $0.140 |
| 28 | 10.7 | $37.50 | $0.125 |
| 35 | 8.6 | $30.00 | $0.100 |
| 45 | 6.7 | $23.33 | $0.078 |
| 55 | 5.5 | $19.09 | $0.064 |
The MPG trap: savings are not linear
Here is the counter-intuitive part. Going from 15 to 20 MPG saves more fuel than going from 35 to 55, even though the second improvement is four times larger in MPG terms.
| MPG | Annual cost | Saved by the next step up |
|---|---|---|
| 15 | $2,800 | — |
| 20 | $2,100 | $700 |
| 25 | $1,680 | $420 |
| 30 | $1,400 | $280 |
| 40 | $1,050 | $350 |
| 50 | $840 | $210 |
The reason is that MPG is a ratio with distance on top: fuel used is the reciprocal, so improvements at the bottom of the range matter far more. Replacing a 15 MPG truck with a 20 MPG one saves more fuel than replacing a 35 MPG car with a 55 MPG hybrid. This is exactly why some countries quote litres per 100 km instead, which is linear in fuel and makes the comparison obvious.
What actually moves real-world MPG
| Factor | Effect |
|---|---|
| Driving 75 mph instead of 65 | −10 to 15% |
| Aggressive acceleration and braking | −10 to 30% |
| Roof box or rack | −10 to 25% at highway speed |
| Tyres 25% under-inflated | −2 to 3% |
| Cold weather, short trips | −10 to 20% |
| 100 lb of extra cargo | −1 to 2% |
| Air conditioning in heavy traffic | −5 to 20% |
Aerodynamic drag rises with the square of speed, which is why the highway-speed penalties are the largest ones on that list. An empty roof rack left on all year is one of the most expensive pieces of forgetfulness available to a driver.
Planning a road trip
- Use round-trip distance. The commonest error on this page is budgeting one way.
- Use your highway figure, not combined, for a long motorway drive — it is usually the higher number.
- Add 10% for detours, traffic, terrain and air conditioning.
- Fuel prices vary by state, sometimes by a dollar a gallon on a long route. Budget the higher end.
- Compare against flying honestly by including only fuel here — the true driving cost per mile including depreciation and maintenance is roughly double the fuel figure.
Measure your actual economy with the MPG calculator rather than trusting the sticker, and compare against an electric vehicle with the EV charging cost calculator.
How this fuel cost calculator works out its numbers
Trip cost is distance ÷ MPG × price per gallon. Gallons needed is distance ÷ MPG, and cost per mile is trip cost ÷ distance.
- Fuel only. Depreciation, maintenance, tyres and insurance are excluded, and together they typically double the true cost per mile.
- Enter round-trip distance for a return journey. The calculator has no way to know which you mean.
- Use your real measured MPG rather than the window sticker, which is derived from standardised test cycles and is commonly optimistic for city driving.
- US gallons. An imperial gallon is about 20% larger, so UK MPG figures are not comparable without converting.
- The efficiency-factor table gives typical published ranges; actual effects vary considerably with vehicle, load and conditions.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Fuel Cost Calculator: frequently asked questions
How do I calculate gas cost for a trip?
Divide the distance by your MPG to get gallons, then multiply by the price per gallon. A 300-mile trip at 28 MPG and $3.50 a gallon is 10.7 gallons and $37.50.
What's a typical cost per mile for fuel?
About $0.14 a mile at 25 MPG with $3.50 gas, or $0.10 at 35 MPG. That is fuel only — total running cost including depreciation and maintenance is roughly double.
Why does improving low MPG save more fuel?
Because fuel used is the reciprocal of MPG. Going from 15 to 20 MPG saves $700 a year at 12,000 miles, while going from 35 to 55 saves less, despite being a four times larger improvement in MPG terms.
How much does speed affect fuel economy?
Driving 75 instead of 65 mph typically costs 10 to 15% of your economy, because aerodynamic drag rises with the square of speed. It is the single largest controllable factor on a long drive.
Does a roof rack hurt gas mileage?
Considerably — 10 to 25% at highway speed with a loaded box, and a meaningful amount even when empty. Removing it when not in use is one of the cheapest improvements available.
How do I estimate gas for a road trip?
Take the round-trip distance, divide by your highway MPG, and multiply by the expected price. Add about 10% for detours, traffic, terrain and air conditioning, and budget the higher end if crossing state lines.
Do under-inflated tyres really cost fuel?
Yes, but less than commonly claimed: about 2 to 3% at 25% under-inflation. The stronger argument for correct pressure is tyre life and safety rather than fuel.
Why is my winter fuel economy worse?
Cold engines run rich until warm, winter fuel blends carry slightly less energy, tyre pressure drops in the cold, and short trips never reach efficient temperature. A 10 to 20% drop is normal.