How to Calculate Your Car's Real MPG
Miles per gallon (mpg) shows how far your car travels on one gallon of gas. Measuring it yourself is easy and shows your real fuel economy.
The formula
MPG = miles driven ÷ gallons used. If you drove 400 miles and used 12.5 gallons, your fuel economy is 400 ÷ 12.5 = 32 mpg.
How to measure it
- Fill the tank completely and note the odometer or reset the trip meter to zero.
- Drive as you normally would until the tank is at least half empty.
- Fill the tank completely again at the same kind of pump.
- Write down the gallons the pump shows and the miles on the trip meter.
- Divide miles by gallons.
Improve accuracy
One tank can be misleading because pumps shut off at slightly different levels. Repeat the test over three tanks and average the results. Compare similar kinds of driving, because city and highway numbers can differ a lot.
Gallons per 100 miles
Another way to show efficiency is gallons per 100 miles = gallons ÷ miles × 100. At 32 mpg, a car uses 3.13 gallons per 100 miles. This number is easier to compare because it shows fuel use directly. Going from 20 mpg to 25 mpg saves more fuel than going from 40 mpg to 45 mpg.
Converting mpg to L/100 km
Many countries show fuel use as liters per 100 kilometers. Use L/100 km = 235.215 ÷ mpg (US gallons). At 32 mpg that is about 7.35 L/100 km. The conversion also works in reverse, since mpg = 235.215 ÷ L/100 km.
Why your mpg differs from the sticker
Official ratings come from standard tests. Real fuel economy changes with:
- Driving speed and acceleration
- Short trips and cold starts
- Cold weather and winter fuel blends
- Tire pressure and load
- Air conditioning and heating
- Terrain, wind and traffic
Typical ranges
Many gasoline cars get roughly 25 to 35 mpg in mixed driving. Hybrids often do better, and large trucks and SUVs often do worse. Check the exact rating for your model at fueleconomy.gov, and compare it with your measured figure.
If your mpg drops suddenly
A sudden fall may mean low tire pressure, a dragging brake, a dirty air filter, an engine problem, or simply colder weather. If it stays low, have a mechanic check the car.
Use your mpg
Once you know your mpg, use the Gas Cost Calculator to estimate trip costs. For more detail, read how to calculate trip gas cost. Or let the MPG Calculator do the division for you.
Example with two tanks
Tank 1: 310 miles on 10.5 gallons is 29.5 mpg. Tank 2: 295 miles on 9.8 gallons is 30.1 mpg. For the average, add the miles and the gallons first: 605 miles ÷ 20.3 gallons = 29.8 mpg. Do not average the two mpg figures, because tanks with different distances should count differently.
Fuel cost per mile
Divide the price per gallon by your mpg. At $3.40 a gallon and 30 mpg, fuel costs about 11.3 cents per mile. This number helps you compare different cars or decide whether a long commute is worth it.
Frequently asked questions
What is a good mpg?
It depends on the vehicle type. Compare with similar vehicles.
Does the dashboard mpg display match the pump?
It is often a little optimistic. The pump method is more reliable.
How often should I check?
A few times a year, or whenever you suspect a problem.