How it works
The most accurate way to measure fuel economy at home is the fill-to-fill method. Fill the tank until the pump clicks off, reset the trip odometer, drive normally, then fill up again the same way. The miles on the trip meter divided by the gallons pumped on the second fill is your MPG for that tank.
MPG = miles driven ÷ gallons used
From MPG the calculator derives the other common measures using exact conversion factors: one mile is 1.609344 kilometers and one U.S. gallon is 3.785411784 liters.
L/100 km = 235.2146 ÷ MPG km/L = MPG × 1.609344 ÷ 3.785411784
The 235.2146 constant is 100 × 3.785411784 ÷ 1.609344. Notice that liters per 100 km runs the opposite way to MPG: a smaller number means a more efficient car.
If you add a fuel price, cost per mile is price ÷ MPG and the cost of the fill-up is gallons × price. If you also add your yearly mileage, annual gallons are annual miles ÷ MPG, and annual fuel cost is annual miles × cost per mile. Monthly cost is that figure divided by twelve.
Methodology notes
- Results use U.S. gallons. Imperial (UK) gallons are about 20% larger, so UK MPG figures are higher for the same car.
- One tank is a small sample. Averaging three or more fill-ups smooths out differences in how full the pump leaves the tank.
- The trip computer in many cars reads a little optimistic. The fill-to-fill method measures fuel actually pumped, so it is the better reference.
Worked examples
Commuter sedan: 342.6 miles on 11.4 gallons, $3.29 gas, 12,000 miles a year
Fuel economy is 342.6 ÷ 11.4 = 30.05 MPG, which is 7.83 L/100 km or 12.78 km/L. The fill-up cost $37.51 and each mile costs $0.11. Over 12,000 miles the car will burn about 399.3 gallons, roughly $1,313.70 a year or $109.47 a month.
Pickup truck: 285 miles on 14.2 gallons, $3.49 gas, 15,000 miles a year
That works out to 20.07 MPG (11.72 L/100 km), $0.17 a mile and $2,608.32 a year in fuel. If better tire pressure and gentler driving lifted the same tank to 327.8 miles, economy would rise to 23.08 MPG and the yearly fuel bill would drop to $2,268.10, a saving of $340.22.
Why your MPG differs from the EPA rating
Official ratings come from standardized lab cycles that can’t match every driver. Short trips are hard on economy because the engine spends much of the drive warming up. Cold weather thickens fluids and, in many areas, winter-blend fuel has slightly less energy per gallon. Aggressive acceleration, long idling, roof racks, towing and under-inflated tires all pull the number down. Hybrids, by contrast, often beat their ratings in slow traffic, where they can run on electric power.
Small MPG gains are worth more on thirsty vehicles
Because fuel used is miles divided by MPG, a one-MPG improvement saves far more fuel on a truck than on an efficient compact. Driving 12,000 miles, going from 15 to 16 MPG saves 50.0 gallons, while going from 40 to 41 MPG saves just 7.3 gallons. That is why some people prefer gallons per 100 miles, or L/100 km, when comparing vehicles: those measures move in step with the fuel you actually buy.
Tracking fuel economy over time
Keep a simple log with the date, odometer reading, gallons and price at every fill-up. A sudden drop of 10% or more that lasts several tanks can be an early warning of a dragging brake, a failing oxygen sensor, a clogged air filter or low tire pressure. A steady log also gives you a dependable MPG for planning road trips with our gas cost calculator and for estimating fuel in the ownership cost calculator.
Tips for better fuel economy
- Keep tires at the pressure printed on the door-jamb placard and check them monthly while cold.
- Accelerate smoothly and look ahead so you brake less.
- Combine errands; a warm engine is more efficient than several cold starts.
- Remove unused roof boxes, bike racks and heavy items from the trunk.
- Follow the maintenance schedule for air filters, spark plugs and the recommended grade of oil.
Frequently asked questions
How do I calculate miles per gallon?
Fill the tank, reset the trip odometer, drive, then refill. Divide the miles on the trip meter by the gallons it took to refill. For example, 342.6 miles on 11.4 gallons is 30.05 MPG.
How do I convert MPG to L/100 km?
Divide 235.2146 by the MPG figure. 30 MPG is about 7.84 L/100 km. To go the other way, divide 235.2146 by the L/100 km value.
What is a good MPG?
It depends heavily on the vehicle class and powertrain: small cars and hybrids rate far higher than large SUVs and pickups. The most useful benchmark is the official EPA rating for your exact model, year and engine, which you can look up and compare by class at fueleconomy.gov.
How do I work out fuel cost per mile?
Divide the price per gallon by your MPG. At $3.29 a gallon and 30 MPG, each mile costs about 11 cents in fuel.
Why does my car’s display show a different MPG?
Trip computers estimate fuel use from injector timing and are often a few percent optimistic. The fill-to-fill method measures fuel actually pumped, so it is the more reliable figure.
Does this calculator use U.S. or UK gallons?
U.S. gallons of 3.785 liters. A UK (imperial) gallon is 4.546 liters, so to compare with a UK figure multiply U.S. MPG by about 1.201.
How many fill-ups should I average?
At least three. Pumps shut off at slightly different levels, so a single tank can be off by a few percent. Add the miles and gallons from several tanks together and divide the totals.