MPG Calculator divides the miles on your trip meter or odometer by the fuel it took to refill, then shows L/100 km, fuel cost per mile, the gap to the EPA rating, and CO2 per mile.
How the MPG Calculator Measures Fuel Economy
The MPG Calculator works out your real fuel economy from the distance you drove and the fuel it took to refill the tank. Drivers use it to track a car’s health, check it against the EPA sticker, and see what each mile costs.
Enter the distance from your trip meter, or two odometer readings from back-to-back fill-ups. Add the fuel it took to refill, the price you paid, and the fuel type.
Distance can be miles or kilometers, and fuel can be US gallons, liters or UK gallons. The main result comes out in MPG (US), L/100 km or MPG (UK), matching the fuel unit you pick.
The MPG Calculator Formula
Miles per gallon is the distance driven divided by the fuel used.
$$\text{MPG} = \frac{\text{Miles driven}}{\text{Gallons to refill}}$$
With the defaults, 268 miles on 12.7 gallons works out to 21.1 MPG.
In odometer mode, the MPG Calculator subtracts the first reading from the second to get the distance. The default readings of 30,455 and 30,723 give the same 268 miles.
A common mistake is dividing by the tank size instead of the fuel it took to refill. The number that counts is what went into the tank at the pump, not how much the tank holds.
Filling Up the Same Way Each Time
The fill-up method is where most real-world errors start. Fill the tank, note the odometer or reset the trip meter, and drive as usual.
At the next fill-up, stop at the pump’s first automatic click, just as you did the first time. Topping off one time and not the other changes the fuel figure and skews the result.
The fuel you pump at the second fill-up is the fuel you burned since the first. That is the number to enter.
Averaging Several Tanks
One tank can be thrown off by weather, a long idle or a different route. Tracking three or more gives a steadier figure.
To combine them, add up all the miles and all the gallons, then divide once. Averaging the MPG of each tank gives the wrong answer when the tanks are different sizes.
Say one tank covered 300 miles on 10 gallons, or 30 MPG, and the next covered 100 miles on 5 gallons, or 20 MPG. The average of the two figures is 25 MPG, but the real result is 400 miles on 15 gallons, or 26.7 MPG.
The MPG Calculator handles that directly. Enter the total miles and total gallons across all the tanks.
MPG, L/100 km and UK MPG
The first card restates your result in the other common units. At the defaults, 21.1 MPG (US) equals 11.15 L/100 km, 8.97 km/L and 25.34 MPG (UK).
$$\text{L/100 km} = \frac{235.215}{\text{MPG (US)}}$$
The UK gallon holds 4.54609 liters against 3.785 for the US gallon. So the same car shows a higher MPG figure in the UK, about 20% higher.
If you track your car in km/L from logged liters, the liter to km calculator covers that conversion on its own.
Fuel Cost per Mile
The second card turns the fill-up into cost. At $3.50 a gallon, the default 12.7 gallons cost $44.45, which is about $0.166 per mile or $165.86 per 1,000 miles.
That is fuel only. For the full cost of owning the car, including insurance, maintenance and depreciation, the cost per mile calculator adds the rest.
To price fuel for a distance you have not driven yet, the fuel cost calculator works forward from your MPG.
Comparing With the EPA Rating
In the MPG Calculator, the third card compares your result with a second figure. Pick the EPA combined rating, your previous tank, or another vehicle.
At the defaults, 21.1 MPG sits 12.1% below an EPA combined rating of 24. Over 1,000 miles, that gap costs 5.72 extra gallons, or $20.02 at $3.50.
The alert turns yellow when you are more than 5% under the EPA figure and green when you are more than 5% over it. Short trips, high speeds, cold weather, idling and heavy loads all pull real mileage under the sticker.
The EPA rating comes from standard lab test cycles, so treat it as a reference point rather than a promise. A gap of a few percent is normal.
CO₂ From Your Driving
The fourth card estimates tailpipe carbon dioxide. The EPA puts it at about 8,887 grams of CO₂ per gallon of gasoline and 10,180 grams per gallon of diesel.
$$\text{CO}_2\ \text{per mile} = \frac{8{,}887\ \text{g}}{\text{MPG}}$$
At 21.1 MPG on gasoline, that is about 421 grams per mile, and the default tank produced about 249 lbs of CO₂. The EPA says the average passenger vehicle emits about 400 grams per mile, so this car runs 5.3% above it.
Diesel makes more CO₂ per gallon, but diesel vehicles often get more miles per gallon. Set the fuel type so the card uses the right factor.
Limits the Tool Checks
Distance, fuel and the comparison figure must be greater than zero. Fuel price can be zero, but not negative.
In odometer mode, the second reading must be higher than the first. If it is not, the MPG Calculator flags the entry and explains why.
Changing the fuel unit converts the fuel amount, the price and the comparison figure together. That keeps a US gallon price from being read as a price per liter.
Mileage Mistakes to Avoid
Entering an EPA highway rating as the comparison in the MPG Calculator makes mixed driving look poor. Use the combined figure on the window sticker or at fueleconomy.gov.
Leaving the fuel unit on gallons after filling up in liters cuts MPG to about a quarter of its real value. Match the unit on your receipt.
Reading MPG from a fill-up that followed a partial fill mixes two tanks. Only use tanks that both started and ended at a full fill.
Gas Mileage Questions
How do I calculate MPG by hand?
Fill the tank and note the odometer. Drive until you need fuel, then fill up again and note the gallons pumped and the new odometer reading. Subtract the old reading from the new one, and divide by the gallons. For example, 268 miles on 12.7 gallons is 21.1 MPG.
Why is my MPG lower than the EPA rating?
EPA ratings come from standard lab tests designed to mimic typical driving. Your results change with speed, weather, short trips, idling, cargo, tire pressure and terrain. A few percent below the sticker is common. If you are well below it across several tanks, check tire pressure and look for a change in how the car is driven.
What is good gas mileage?
It depends on the type of vehicle. The EPA puts the average gasoline vehicle on the road at about 22.2 MPG. The EPA Automotive Trends Report puts new model year 2024 vehicles at a record 27.2 MPG. Compare your figure with the EPA combined rating for your own model in the MPG Calculator to see how it is doing.
Does E10 gas lower MPG?
Slightly. The EPA says an ethanol blend gives slightly lower fuel economy than gasoline without ethanol. Tailpipe CO₂ per mile stays similar, because ethanol carries less carbon per gallon. Most US gasoline is already up to 10% ethanol, so your tracked MPG likely reflects it already.
How do I convert MPG to L/100 km?
Divide 235.215 by the MPG (US). A car getting 25 MPG uses about 9.41 L/100 km, and 30 MPG is about 7.84 L/100 km. Unlike MPG, a lower L/100 km figure is better. The first result card does this conversion automatically.
How far can I drive on the fuel I have?
Multiply the fuel in your tank by your MPG. At 21.1 MPG, 10 gallons covers about 211 miles to empty. The fuel distance calculator runs the same math and keeps a reserve so you don’t plan to run dry.