The formula behind the number
The math is intentionally simple. In U.S. units: Trip cost = (Distance ÷ MPG) × Price per gallon. In metric: Trip cost = (Distance × L/100km ÷ 100) × Price per litre. The dividing quantity is always fuel economy — the higher your MPG (or the lower your L/100km), the less you spend. Everything else is arithmetic.
Worked example
You're driving 500 miles from Denver to Salt Lake City in a car that averages 32 MPG. Gas at the highway stations is running $3.60 per gallon. Gallons needed: 500 ÷ 32 ≈ 15.6. Cost: 15.6 × $3.60 ≈ $56.25. Cost per mile: 3.60 ÷ 32 ≈ 11.25¢. Round trip: about $112.50 in fuel.
| MPG | $3.00/gal | $3.60/gal | $4.20/gal | $5.00/gal |
|---|---|---|---|---|
| 20 MPG | $75.00 | $90.00 | $105.00 | $125.00 |
| 25 MPG | $60.00 | $72.00 | $84.00 | $100.00 |
| 32 MPG | $46.88 | $56.25 | $65.63 | $78.13 |
| 40 MPG | $37.50 | $45.00 | $52.50 | $62.50 |
| 50 MPG (hybrid) | $30.00 | $36.00 | $42.00 | $50.00 |
Figuring out your actual MPG
The window sticker MPG is an EPA estimate under controlled conditions — your real-world number is often 5%–15% lower. To measure it yourself: fill up completely, reset the trip odometer, drive normally until you're near empty, then fill up again. Miles driven ÷ gallons pumped on the second fill-up = your actual MPG. Do this two or three times across seasons and take the average — winter and short trips both drop MPG measurably.
What actually moves the needle
- Speed: fuel economy drops sharply above 55–60 mph. Every 5 mph over 60 costs roughly 7% MPG.
- Aggressive driving: hard acceleration and braking can cut highway MPG by 15%–30%.
- Cold weather: a gasoline engine takes 15–20 minutes to warm up; short cold trips can halve advertised MPG.
- Tire pressure: 10 psi under-inflated tires drop MPG by 3%–4%.
- Roof cargo: an add-on roof box can cost 15%–25% MPG on highway trips.
- AC vs. windows: at highway speed, closed windows plus AC is more efficient than open windows (drag beats compressor load).
Comparing gas cost with an EV
For an apples-to-apples comparison, convert an EV's efficiency to gasoline-gallon equivalents. The EPA uses 33.7 kWh per gallon of gasoline (a GGE). A car rated at 30 kWh/100 miles is equivalent to 33.7 / (30/100) = 112 MPGe. At $0.15/kWh, that 500-mile trip costs 500 × 0.30 × $0.15 = $22.50 — less than half the $56.25 gas cost above. Public DC fast charging at $0.40–$0.60/kWh narrows that gap considerably.
Business mileage reimbursement
For 2026 the U.S. IRS standard business mileage rate is 70¢ per mile. That figure covers fuel, depreciation, maintenance, insurance, and registration — not just gas. Using this calculator you can separate the fuel-only portion (11¢–15¢/mile for most gas cars) from the ownership costs when negotiating a company car allowance or freelance travel reimbursement.
How to use this fuel cost calculator
- Enter the trip distance one way (or the round trip — the tool handles either).
- Enter your car's real-world MPG or L/100 km.
- Enter the current fuel price at the pump.
- Read the total fuel needed, total cost, and cost per mile/km.
- Toggle between U.S. and metric units if your inputs are mixed.
Where this calculator is only an estimate
It assumes constant MPG for the whole trip — a mountainous route or heavy city traffic will use more fuel than the average predicts. Trailer towing or a fully-loaded car can drop MPG by 20%–40%. Ethanol-blended gasoline (E15, E85) has lower energy density than regular unleaded; expect 3%–25% lower MPG depending on blend. For a route split across cities and countries, break the trip into legs and calculate each separately, then sum.
Related calculators
Combine with the auto loan calculator to see the full cost of ownership (payment plus fuel), the salary calculator to gauge what share of your income vanishes at the pump each month, and the inflation calculator to project how a $3.60 gallon compares to prices a decade from now.