MPG Calculator

Miles per gallon (MPG) measures how far a vehicle travels on one gallon of fuel. Metric mode uses liters per 100 kilometers (L/100 km) — lower is more efficient. The fill-up method is the most accurate: reset the trip odometer at a full tank, drive normally, then divide distance by fuel added at the next fill-up.

EPA window-sticker ratings are laboratory estimates. Your real MPG depends on speed, traffic, load, tire pressure, and weather. City driving typically scores lower than highway cruising.

Example: you drove 312 miles and used 10.5 gallons at the pump. MPG = 312 ÷ 10.5 ≈ 29.7 MPG. That equals about 7.9 L/100 km for international comparison.

MPG and L/100 km move in opposite directions, which makes them easy to misread. MPG measures distance per unit of fuel, so higher is better, while L/100 km measures fuel per fixed distance, so lower is better. The two are reciprocals: divide 235.2 by an MPG figure to get L/100 km, and the same constant converts back.

Because MPG is a ratio, equal gains are not equal savings. Going from 15 to 20 MPG saves about 167 gallons over 10,000 miles, while going from 30 to 35 MPG over the same distance saves only 48 gallons. Improving an already-efficient vehicle yields much less fuel than improving a thirsty one.

One fill-up is a noisy sample. Pump auto-shutoff, tank slope, and how firmly you top off can shift a single reading by a mile per gallon or more, so average three or four consecutive tanks before trusting the number. Always fill to the same click point at the same pump if you can.

Trip-computer readings tend to run optimistic by a few percent against hand calculation, because they infer fuel from injector timing rather than measuring what actually went into the tank. Use the pump-and-odometer method as your reference and treat the dashboard figure as a live indicator.

Speed matters more than most drivers expect. Aerodynamic drag rises with the square of speed, so fuel use climbs steeply above roughly 50 mph — the US Department of Energy estimates each 5 mph over 50 costs the equivalent of paying meaningfully more per gallon. Under-inflated tires and a roof box compound the effect.

US mode uses US gallons. UK imperial gallons are ~20% larger — do not mix them when comparing to UK fuel economy figures.