Gas Mileage Calculator
MPG ยท L/100 km ยท Cost per mile ยท EPA rating check
โฝ Enter the distance driven and the fuel added to see your MPG, L/100 km, cost per mile, and yearly fuel cost.
What Is Gas Mileage and What This Calculator Does
Gas mileage is the distance a vehicle travels on a set amount of fuel. In the United States it is measured in miles per gallon (MPG), where a higher number means the car uses less fuel. Most other countries use liters per 100 kilometers (L/100 km), where a lower number is better, or kilometers per liter (km/L).
This gas mileage calculator works out your real fuel economy from two numbers you already have: the distance driven between fill ups and the fuel you pumped. It then converts the result into every common unit, shows your fuel cost per mile or kilometer, projects yearly fuel spending, estimates tailpipe CO2, and compares your figure with the official rating for your vehicle.
Measured mileage is more useful than the window sticker for budgeting, because it reflects your own routes, climate, driving style, and vehicle condition. Tracking it over time also helps you spot problems such as a dragging brake or low tire pressure before they get expensive.
Gas Mileage Formulas
Miles per gallon
Liters per 100 kilometers
Converting between units
Fuel cost per mile
The constant 235.215 comes from the exact unit definitions: one mile is 1.609344 km and one US gallon is 3.785411784 liters. The calculator uses these exact values and rounds only the displayed figures.
MPG to L/100 km Conversion Chart
Use this chart to convert gas mileage between US MPG, UK MPG, L/100 km, and km/L without a calculator.
| US MPG | UK MPG | L/100 km | km/L |
|---|---|---|---|
| 15 | 18.0 | 15.7 | 6.4 |
| 20 | 24.0 | 11.8 | 8.5 |
| 25 | 30.0 | 9.4 | 10.6 |
| 30 | 36.0 | 7.8 | 12.8 |
| 35 | 42.0 | 6.7 | 14.9 |
| 40 | 48.0 | 5.9 | 17.0 |
| 50 | 60.0 | 4.7 | 21.3 |
| 60 | 72.1 | 3.9 | 25.5 |
How to Calculate Gas Mileage Manually (Full Tank Method)
The full tank method is the standard way to measure real MPG. It needs only the odometer and a pump receipt, and it is what this calculator's odometer mode reproduces.
- Fill the tank completely. Stop at the first automatic click of the nozzle. Write down the odometer reading or reset trip meter A.
- Drive normally. Use the car as you usually do until the tank is at least half empty. A longer interval reduces the error from how full the tank is.
- Fill the tank again. Use the same pump if possible and stop at the first click again. Note the gallons or liters pumped and the new odometer reading.
- Subtract the readings. End odometer minus start odometer gives the distance driven.
- Divide. Distance divided by fuel added gives MPG or km/L. For L/100 km, divide liters by kilometers and multiply by 100.
Worked example
A driver fills up at 12,000 miles, drives a week of mixed commuting, and refills at 12,350 miles. The pump shows 10.5 gallons at $3.50 per gallon, so the fill up cost $36.75.
Distance is 12,350 โ 12,000 = 350 miles. Gas mileage is 350 รท 10.5 = 33.3 MPG. In metric terms that is 563.3 km on 39.75 liters, or 7.06 L/100 km. Fuel cost per mile is $36.75 รท 350 = $0.105. At 12,000 miles per year the car would use about 360 gallons and cost about $1,260 in fuel.
If the official combined rating is 32 MPG, the measured 33.3 MPG is 4 percent above it, which is within normal variation. Repeating the measurement over three to five tanks and dividing total distance by total fuel gives a more reliable average than any single tank.
What Is Good Gas Mileage?
There is no single number that counts as good gas mileage, because a full size pickup and a compact hybrid are designed for different jobs. The fairest benchmark is the official EPA rating for your exact year, make, model, and engine, which you can look up on fueleconomy.gov and enter in the rated economy field above.
The calculator grades your result against that rating:
- On track: within 5 percent of the rating or better. Normal for mixed driving.
- Below rating: 5 to 15 percent lower. Often explained by weather, short trips, or traffic.
- Well below: more than 15 percent lower. Worth checking tires, loads, and maintenance.
These bands are practical guides chosen for this tool, not an official EPA classification. EPA ratings come from standardized laboratory test cycles, so no driver matches them exactly.
Gas mileage for city vs highway driving
Most gasoline cars get better mileage on the highway, because steady speeds avoid repeated acceleration. Hybrids often reverse this pattern, because regenerative braking recovers energy in stop and go traffic. Measure each driving pattern separately if you want to see the difference for your own car.
Why Your MPG Is Lower Than Expected
According to fueleconomy.gov, aggressive driving with rapid acceleration and hard braking can lower gas mileage by roughly 15 to 30 percent at highway speeds and 10 to 40 percent in stop and go traffic. Other common causes include:
- Cold weather and short trips. An engine uses more fuel until it reaches operating temperature.
- High speed. Aerodynamic drag rises quickly with speed, so mileage usually drops above highway cruising speeds.
- Extra weight and roof cargo. Roof boxes and bike racks add drag even when empty.
- Underinflated tires. Keep tires at the pressure printed on the driver door jamb label.
- Idling. An idling engine uses fuel while the odometer reads zero distance.
- Maintenance issues. Dragging brakes or a faulty sensor can cut mileage noticeably.
Common Mistakes When Measuring MPG
- Not filling to the same level. Topping off past the first click on one visit but not the next skews a single tank result. Stop at the first click every time.
- Using a partial fill up. If you add only a few gallons, the next complete fill up must include all fuel since the last full tank.
- Mixing units. Entering liters while the calculator is set to gallons inflates MPG by a factor of about 3.8. Check the unit toggle first.
- Confusing US and UK MPG. A UK gallon is 4.546 liters, so the same car shows about 20 percent higher MPG in UK units.
- Trusting the dashboard display alone. Onboard computers estimate fuel flow and can read several percent high. Pump receipts are the reference.
- Judging from one tank. Weather, traffic, and a single road trip can move one reading a lot. Average several tanks.
CO2, Cost, and Related Calculations
Burning fuel releases carbon dioxide in proportion to the amount burned. The US EPA uses about 8,887 grams of CO2 per gallon of gasoline and about 10,180 grams per gallon of diesel. The calculator multiplies your yearly fuel use by these factors, so 360 gallons of gasoline produces about 3,200 kg of CO2.
To estimate the cost of a planned trip from a known MPG, use the fuel cost calculator. To compare the full cost of owning a vehicle, combine your yearly fuel figure with the payment from the auto loan calculator. The conversion calculator handles other distance and volume units.
Electric vehicles are rated in MPGe, which counts 33.7 kWh of electricity as equal to one gallon of gasoline. This calculator measures liquid fuel only.
Frequently Asked Questions
Sources & References
- U.S. Department of Energy and U.S. EPA, Fuel Economy Guide and vehicle ratings, fueleconomy.gov. https://www.fueleconomy.gov/
- fueleconomy.gov, Driving More Efficiently. https://www.fueleconomy.gov/feg/driveHabits.jsp
- U.S. EPA, Greenhouse Gas Emissions from a Typical Passenger Vehicle (CO2 per gallon factors). https://www.epa.gov/greenvehicles/greenhouse-gas-emissions-typical-passenger-vehicle
- NIST, Handbook 44 Appendix C, General Tables of Units of Measurement (mile and gallon definitions). https://www.nist.gov/pml/owm/nist-handbook-44-current-edition
Results are estimates based on the values entered. Fuel prices change often, so enter your current local price. Reference data reviewed: 2026.