⛽ Fuel Mileage Calculator

How to Use This Calculator

Enter the miles driven and gallons used from a fill-up cycle, plus the fuel price per gallon, then hit Calculate MPG. The headline is your miles per gallon; the details show L/100km, fuel cost per mile, and, if you entered trip miles, the total trip fuel cost and gallons needed. For the most accurate MPG, use the full-tank method over several tanks: fill completely, record the odometer, drive, fill completely again, and divide miles by the second fill's gallons.

The Method: Division, Then Multiplication

MPG is a simple ratio, and every derived figure is one more arithmetic step. The L/100km conversion uses the exact constant 235.214, and remember the inversion: bigger MPG is better, smaller L/100km is better. Cost per mile is the number that matters for budgets: at 32 MPG and $3.50 gas, each mile costs about 11 cents in fuel. Trip cost multiplies that by distance, with a suggested 10 percent buffer for detours. The full-tank method matters because gas station pumps shut off at slightly different fill levels; measuring over one tank can be off by half a gallon, but over five tanks the error washes out.

Worked Example

You drive 320 miles and the fill-up takes 10 gallons at $3.50 per gallon, and you are planning a 1,000-mile road trip. MPG = 320 / 10 = 32. L/100km = 235.214 / 32 = 7.35. Cost per mile = 3.50 / 32 = $0.109. Trip gallons = 1,000 / 32 = 31.25. Trip cost = 31.25 x 3.50 = $109.38, call it $120 with buffer. Enter those numbers and the calculator reproduces each figure. The sanity check: 32 MPG at $3.50 means each dollar buys about 9 miles, so 1,000 miles needs about $109. Same answer, different path.

What Actually Moves the MPG Needle

Speed dominates: aerodynamic drag rises with the square of speed, so 75 mph can cost 15 to 20 percent versus 60 mph. Tire pressure is the free win; underinflation by 5 PSI measurably hurts economy. Aggressive acceleration and braking waste the energy you just paid for; smooth driving is worth 10 percent or more in city cycles. Roof boxes and racks are MPG killers at highway speed. Idling gets zero MPG by definition, so modern start-stop systems earn their keep. Weight matters least for steady highway cruising and most in stop-and-go. Fix the big ones first: speed, tires, and driving style.

Fuel Cost Versus True Cost

Fuel is roughly a quarter to a third of true per-mile cost. The IRS business rate of 76 cents per mile (second half of 2026) bundles depreciation, insurance, maintenance, and fuel into one number, which is why it dwarfs the 11-cent fuel figure. Depreciation is the silent giant on new cars; maintenance grows on old ones. When comparing vehicles, compute five-year total cost: purchase price difference plus fuel plus insurance. A hybrid's fuel savings are real but only part of the ledger, and the same math that sells hybrids can also justify keeping a paid-off efficient car.

Hypermiling: The Techniques That Work

Hypermiling techniques can lift real-world mileage 15 to 30 percent without vehicle changes. Ranked by payoff: slow down (aerodynamic drag rises with the square of speed; 55 vs 75 mph can save 20+ percent), anticipate stops (every brake press wastes the fuel that built your speed), keep tires at the door-sticker pressure (underinflation costs 1-2 percent per 5 PSI), and cut idle time (modern engines need no warm-up; restart beats idling past 30 seconds). Cruise control helps on flat highways but hurts on hills, where it downshifts aggressively; a skilled foot beats it there. Drafting trucks closely is the technique to skip: the fuel savings are real but the safety cost is obscene. Track mileage per tank with this calculator; the feedback loop is what makes the habits stick.

MPGe: Comparing With Electric Cars

MPGe (miles per gallon equivalent) lets EVs and gas cars share a metric: 33.7 kWh of electricity equals the energy in a gallon of gasoline. A typical EV rates 100+ MPGe, reflecting the electric motor's far higher efficiency, but MPGe measures energy, not cost or emissions. For cost comparison, compute cents per mile: EV at 13 cents/kWh and 3.5 miles/kWh costs 3.7 cents/mile; a 30 MPG gas car at $3.50/gallon costs 11.7 cents/mile. Emissions depend on your grid: coal-heavy regions narrow the EV advantage, clean grids widen it. When comparing, use your actual electricity rate (check time-of-use plans) and your real gas price. The calculator's per-mile cost figure is the number that decides the household budget, whatever the powertrain.

The Five-Cent Rule: Translating MPG Into Trip Money

MPG is abstract until it is dollars. Take a 500-mile road trip at $3.50 a gallon. At 25 MPG you burn 20 gallons: $70. At 35 MPG you burn 14.3 gallons: $50. The 10-MPG gap is worth exactly $20 on this trip, which is a useful anchor: for typical gas prices, every 5 MPG is worth roughly a nickel a mile. A car that gets 30 instead of 25 saves about 5 cents per mile, or $750 a year at 15,000 miles.

Run that against the car payment before you upgrade for economy alone. Trading a paid-off 25-MPG car for a $400-a-month 35-MPG replacement saves $750 a year in fuel and costs $4,800 in payments. The math only favors the efficient car when the old one needs replacing anyway, or when fuel prices spike. Efficiency is worth real money. It is just rarely worth a car payment by itself.

The Tire Pressure Dividend

The cheapest fuel-economy mod ever made is a $5 tire gauge. Under-inflated tires raise rolling resistance, and the Department of Energy's long-standing figure is that every 1 PSI below spec costs about 0.2% in fuel economy. Four tires 5 PSI low: roughly 4% worse mileage, which on a 25-MPG car is a full mile per gallon gone, silently, for months. Check pressures monthly, cold, against the sticker on the driver's door jamb, not the number on the tire sidewall, which is the maximum, not the target.

While you are down there, look at the roof. A roof rack with nothing on it costs 2 to 8% in highway fuel economy from drag, and a cargo box is worse. Take the rack off when the trip is over and the car pays you back every commute. Aerodynamics is free horsepower in reverse: removing drag is the only mod that improves both mileage and quietness at once.

The Winter MPG Penalty

Expect your mileage to drop 10 to 20% in winter, and do not blame the car. Cold air is denser, which raises aerodynamic drag. Winter gasoline blends contain slightly less energy per gallon. The engine runs rich longer while warming up, tire pressure drops with temperature, and short trips never reach efficient operating temperature at all. A 30-MPG car becoming a 25-MPG car in January is physics, not a repair bill. Track seasonal averages separately or the winter numbers will make you chase problems that do not exist.