Gas Mileage Calculator (MPG & Trip Fuel Cost)
Calculate fuel economy in MPG and L/100km, gallons or liters needed, total trip fuel cost, and cost per mile from distance, MPG, and gas price.
Gas Mileage — interactive calculator, computed instantly in your browser.
Gas Mileage and Trip Fuel Cost: The Numbers Behind Every Fill-Up
Distance ÷ MPG × Price = What You Pay at the Pump
Whether you are budgeting a road trip or sanity-checking a used car, gas mileage reduces to three numbers: distance, fuel economy, and price per gallon. This calculator chains them — gallons needed, total cost, cost per mile — and converts your MPG into the metric figures (L/100 km, km/L) that the rest of the world quotes.
1. The Trip Fuel Formulas
- Worked default: 500 mi ÷25 mpg =20 gallons (75.71 L) × $3.50 = $70.00, or $0.14 per mile
- Round trip doubles everything:40 gallons, $140 — easy to forget when comparing flying versus driving
- Cross-country runs reward MPG in non-linear ways: the same500 miles at18 mpg costs $97.22 instead — a39% jump from a28% efficiency drop
- Real consumption differs from the window sticker: city stop-and-go drags economy down, steady highway cruising lifts it
2. Converting Between the World's Fuel Units
- 25 mpg US = 9.41 L/100 km =10.63 km/L =30.02 mpg Imperial
- The two scales run opposite: Americans celebrate high mpg, Europeans celebrate low L/100km — same physics, inverted axis
- Canadian readers:Canada quotes L/100km but often prices fuel per liter — multiply your L/100km trip volume by the pump price directly
- Quick mental check: mpg × L/100km ≈235 — if yours wildly misses, one of the two numbers was entered in the wrong system
3. Squeezing More Miles From Each Tank
- Speed is the biggest lever: aero drag rises with the square of speed — dropping from75 to65 mph can save5%–10% on the highway
- Tire pressure (check monthly), gentle acceleration, and removing roof racks at highway speeds each add1%–3%
- Idling gets zero miles per gallon — modern start-stop systems and avoiding traffic lights beat almost any driving "hack"
- EV comparison: at3.5 mi/kWh and $0.15/kWh an electric car costs about 4.3¢/mile versus14¢ for25-mpg gas — roughly a third, before maintenance
🔗 Road-Trip Cross-References
Fuel math is unit math in disguise — every conversion you need for the trip lives in our 🔁 Conversion Hub, from distances to speeds.
Pair the gallons with a time estimate: divide the miles by your average speed in the time calculator to know exactly when you arrive.
Frequently Asked Questions (Gas Mileage)
Divide the trip distance by your vehicle's MPG:500 miles at25 mpg needs20 gallons (75.7 liters). Multiply by the pump price for total cost — $3.50/gal gives $70. Add10% as a buffer for detours, idling, and terrain the estimate cannot see.
Divide235.21 by your US MPG figure:25 mpg →235.21 ÷25 =9.41 L/100km. The metric figure is consumption (fuel per distance, lower is better) while mpg is efficiency (distance per fuel, higher is better) — they are reciprocal measures of the same thing.
Sticker numbers come from standardized test cycles that flatter most driving: cold starts, short trips, traffic, aggressive acceleration, roof cargo, tires underinflated, and idling all push real MPG below the rating. Reset the trip computer and average a full mixed tank for a true figure.
Run both: driving cost = distance ÷ MPG × price ×2 for the return (add lodging and food on long hauls), while airfare multiplies by passenger count plus airport ground transport. The calculator gives you the honest per-mile driving number to put against the tickets.