Gas Mileage Calculator

The Gas Mileage Calculator calculates real-world fuel economy from fill-up data. Enter the distance driven, or enter two odometer readings, then add the fuel amount from the pump. The calculator returns MPG (US), MPG (Imperial), L/100km, km/L, mi/L, trip cost, estimated tank range and CO2 output. It is built for drivers who want the actual fuel consumption of a car, truck, motorcycle, fleet vehicle or rental vehicle instead of relying only on a dashboard estimate.

Gas Mileage Formula

For the most accurate result, use the full-tank method: fill the tank completely, drive normally, then fill the tank again and use the fuel amount from the second fill-up.

How to Use the Gas Mileage Calculator

1. Start with a full tank: Fill the tank completely and reset the trip meter, or write down the odometer reading.
2. Drive normally: Use the vehicle as usual until the next refill. A longer distance usually gives a more stable result.
3. Refill and enter fuel added: Fill the tank again at the same cutoff style, then enter the distance driven or both odometer readings plus the fuel amount shown on the pump.
4. Review actual economy: Read the calculated MPG, L/100km, km/L, cost and CO2 estimates, then compare the result with the benchmark band.

Common Fuel Economy Units

Tips for Accurate Fill-Up Math

Why Actual Gas Mileage Changes

Real-world fuel consumption changes with speed, temperature, traffic, tire pressure, engine maintenance, payload, terrain and driving style. Stop-and-go city driving usually lowers MPG, while steady moderate-speed highway driving often improves it. Cold weather, short trips and aggressive acceleration can also raise L/100km because the engine spends more time outside its efficient operating range.

Frequently Asked Questions

How do I calculate gas mileage after filling up?

Fill the tank, reset the trip meter, drive normally, then fill the tank again. Divide miles driven by US gallons added to get MPG. For metric fuel consumption, divide liters added by kilometers driven and multiply by 100 to get L/100km.

Is MPG or L/100km better for comparing cars?

Both describe the same fuel use from different directions. MPG and km/L measure distance per unit of fuel, so higher is better. L/100km measures fuel used per fixed distance, so lower is better. L/100km is often easier for cost and emissions comparisons because it scales directly with fuel burned.

Why is my calculated MPG different from the dashboard display?

Dashboard displays estimate fuel flow electronically and can be optimistic or delayed. Fill-up math uses the pump amount and actual distance, but it can vary if the tank is not filled to the same level each time, if the pump shuts off differently, or if the trip distance is very short.

How many fill-ups should I average?

One fill-up gives a useful snapshot, but three to five full-tank records are better. Averaging several tanks smooths out pump cutoff differences, weather, traffic, towing, tire pressure and route changes.

Can I use odometer readings instead of trip distance?

Yes. Leave distance blank and enter the starting and ending odometer readings. The calculator subtracts the start reading from the end reading and uses that distance for the fuel economy calculation.

Additional Resources

• FuelEconomy.gov - U.S. Department of Energy
• U.S. Department of Energy - Driving More Efficiently