Gas Mileage Calculator

Miles per gallon (MPG) is simply distance driven divided by fuel consumed: MPG = miles ÷ gallons. To get an accurate reading, fill the tank, reset your trip odometer, drive normally for a full tank or two, then refill to the same level (auto shut-off click) and divide trip miles by the gallons it took to top off. A single fill-up gives a noisy estimate; averaging 2–3 tanks smooths out variation from terrain, traffic, and weather.

Outside the US, fuel economy is reported in liters per 100 kilometers (L/100 km), which inverts the relationship — lower is better. The conversion is L/100 km = 235.215 ÷ MPG. So 30 MPG = 7.84 L/100 km; 40 MPG = 5.88 L/100 km. The 235.215 constant comes from converting gallons to liters (× 3.785) and miles to kilometers (× 1.609), then inverting.

Cost per mile is the practical metric most drivers actually care about: price per gallon ÷ MPG. At $3.50/gallon and 28 MPG, every mile costs 12.5¢ in fuel. Multiply by your trip distance to estimate fuel cost — this calculator does that automatically in the trip-cost panel.

You fill up after driving 320 miles and the pump shows 11.0 gallons at $3.50/gallon. MPG = 320 ÷ 11 = 29.1. Metric equivalent = 235.215 ÷ 29.1 = 8.08 L/100 km. Cost per mile = $3.50 ÷ 29.1 = $0.120/mile.

Planning a 500-mile road trip in the same vehicle: fuel needed = 500 ÷ 29.1 = 17.2 gallons; total fuel cost = 17.2 × $3.50 = $60.20. Round up 10–15% to account for traffic, A/C use, and detours and you'd budget about $70.

If you switched to a hybrid that gets 48 MPG on the same route: fuel needed = 10.4 gallons, cost = $36.40. The same 500-mile trip costs ~$24 less. Over 12,000 miles/year that's roughly $580 saved on fuel alone.

EPA tests use controlled driving cycles on a dynamometer with no wind, no hills, mild temperature, and a trained driver. Real-world conditions consistently give 10–20% lower MPG than the sticker.

Cold starts: an engine needs 5–10 minutes to reach efficient operating temperature. Short trips (under 4 miles) can cut MPG by 30%+.

A/C and heater: A/C draws 1–4 horsepower from the engine. On a small car, that can shave 10–25% off MPG in summer city driving.

Speed: aerodynamic drag rises with the square of speed. Going 75 mph instead of 65 mph typically costs 10–15% MPG. Going 85 vs 65 can cost 25%.

Driving style: aggressive acceleration and hard braking convert fuel directly into heat through the brakes. Smooth driving alone can recover 10–20% MPG.

Cargo and roof racks: 100 lbs of extra weight ≈ 1% MPG loss; an empty roof rack can cost 2–8% from drag alone.

Inflate tires to spec — under-inflated tires cost ~3% MPG and wear unevenly. Check monthly when cold.

Remove roof boxes, racks, and bike carriers when not in use. The drag penalty is permanent whether they're loaded or empty.

Use cruise control on flat highway stretches; manually feathering the throttle wastes fuel.

Combine errands into one trip with a fully warmed engine instead of three cold starts.

Replace clogged air filters and use the manufacturer's recommended motor oil viscosity — both can move the needle 1–3%.

If you drive 12,000+ miles per year, the spread between a 25 MPG vehicle and a 45 MPG vehicle at $3.50/gallon is about $1,300/year — a meaningful input to next-vehicle decisions.