Convert between fuel economy units, measure your real mileage and compare gasoline and electric cars on cost per mile. Flags the 4 mistakes that most often produce a plausible-but-wrong answer.
Key takeaways
- Convert between mpg and L/100 km by division, and confirm which gallon a rating uses.
- Compare cars on fuel used per distance, not by averaging mpg figures.
- Measure your own mileage over several full tanks, not one.
- Cost per mile is the fuel price divided by the mpg, and for electric cars it is energy per mile times the electricity price.
- Add charging losses and use the electricity rate you will really pay.
Two ways to state the same thing
Fuel economy is quoted in opposite directions. Miles per gallon says how far one gallon takes you, and liters per 100 kilometers says how much fuel a distance takes. A higher mpg is better, and a lower L/100 km is better. Because one is the reciprocal of the other, the conversion is a division, not a simple multiplication.
US and imperial gallons are different
A US gallon is about 3.785 liters, and an imperial gallon is about 4.546 liters, roughly 20 percent larger. The same car therefore has a higher rating in imperial miles per gallon than in US miles per gallon. Thirty US mpg is about 7.84 L/100 km, and thirty imperial mpg is about 9.42 L/100 km. Always confirm which gallon a figure uses.
Why mpg misleads when you compare cars
Equal steps in mpg are not equal savings. Going from 10 to 20 mpg saves more fuel over a distance than going from 20 to 100 mpg: over 100 miles the first step saves 5 gallons and the second only 4. Fuel used per distance, as in L/100 km or gallons per 100 miles, adds up correctly, so averaging mpg ratings across cars or trips gives the wrong result. Convert to fuel per distance before you average.
Measure your own mileage
The rating on the window sticker is a test result, and your car will differ. Fill the tank, note the odometer, drive as usual and fill again. Miles since the first fill divided by the gallons added is your mileage for that tank. Average several tanks, because weather, traffic, load and tire pressure all move a single reading.
Turn mileage into cost per mile
Cost per mile is the price of a gallon divided by the miles per gallon. At $3.50 a gallon and 28 mpg, each mile costs about 12.5 cents. Multiply by the miles you drive each year to see the fuel bill. The same formula lets you compare a car that uses cheaper fuel with one that uses a more expensive one.
Electric cars use kilowatt-hours per distance
An electric car's efficiency is quoted in kilowatt-hours per 100 miles, or in miles per kilowatt-hour, and a converted figure called miles per gallon equivalent lets you compare it with gasoline cars. The cost per mile is the energy per mile times the price of electricity. It is the same arithmetic as gasoline, with a different unit and a different price.
Charging losses are part of the cost
Not all the energy drawn from the wall reaches the battery. Some is lost as heat in the charger and the battery, so the wall draws more than the car's rating suggests. A loss of around 10 percent is common for home charging. The EV charging cost calculator lets you add a loss so the cost reflects what the meter records.
Your electricity rate changes the answer
Home electricity prices vary widely by region and by plan, and time-of-use rates can make overnight charging much cheaper. Public fast charging often costs far more per kilowatt-hour than home charging. Use the rate you will actually pay for most of your charging, and check how it compares with the gasoline price where you live.
What else belongs in a fair comparison
Fuel or energy is only one cost. Compare the purchase price, incentives, insurance, maintenance, depreciation and the cost of a home charger. Electric cars have fewer moving parts and often lower maintenance, while gasoline cars can be cheaper to buy. A total cost of ownership over the years you expect to keep the car tells more than fuel alone.
Driving habits and conditions
Speed, acceleration, cold weather, short trips and heavy loads all reduce efficiency in any car. Cold weather cuts the range of an electric car noticeably and lowers gasoline mileage too. When you estimate a yearly cost, use the mileage you really get in your conditions, not the best rating.
Worked with real numbers
What this looks like in the MPG calculator
The guidance above is easier to judge against figures. Using the MPG calculator worked example, moving miles driven from 280 to 420 changes the miles per gallon from 22.4 to 33.6.
| Miles driven | Miles per gallon | Fuel cost per mile | Liters per 100 km |
|---|---|---|---|
| 280 | 22.4 | $0.16 | 10.5 |
| 315 | 25.2 | $0.14 | 9.3 |
| 350 | 28 | $0.13 | 8.4 |
| 385 | 30.8 | $0.11 | 7.6 |
| 420 | 33.6 | $0.10 | 7 |
Open the MPG Calculator to use your own numbers →
The inputs behind those figures
| Input | Value | Definition |
|---|---|---|
| Miles driven | 350 | Enter the miles driven used in this calculation. |
| Gallons of fuel used | 12.5 | Gallons added at the fill-up that follows the miles driven. |
| Fuel price per gallon | $3.50 | Enter the fuel price per gallon used in this calculation. |
What goes wrong
Common mistakes
Each of these produces an answer that looks reasonable, which is why they survive review. To catch them in comparing fuel economy: mpg, l/100 km and electric cars, rerun the MPG calculator with a different assumption and check whether the result moves in the direction the guidance predicts, since an error that survives that test is usually in one of the inputs and not in the arithmetic.
| The mistake | Why it misleads | Do this instead |
|---|---|---|
| Averaging mpg ratings | Miles per gallon does not add linearly, so an average of two cars misstates their combined fuel use. | Convert to fuel per distance, average that and convert back if you need mpg. |
| Mixing US and imperial gallons | The imperial gallon is about 20 percent larger, so the same car looks more efficient in imperial mpg. | Check the unit before you compare figures from different countries. |
| Trusting one tank | A single fill-up varies with weather, traffic and how full you fill the tank. | Average three or four tanks before you compare with the sticker. |
| Ignoring charging losses | The wall supplies more energy than reaches the battery, so a cost from the rating alone is low. | Add a loss percentage when you calculate the cost of electricity. |
People also ask
Frequently asked questions
How do I convert mpg to L/100 km?
Divide 235.215 by the US mpg figure. Thirty mpg is about 7.84 L/100 km. The fuel economy converter handles the US and imperial gallons.
What is the difference between US and imperial mpg?
The imperial gallon is about 4.546 liters and the US gallon about 3.785, so the same car has a higher number in imperial mpg.
How do I calculate my car's real mpg?
Fill the tank, note the odometer, drive, fill again, and divide the miles by the gallons added. Average several tanks.
How do I calculate cost per mile?
Divide the price per gallon by the miles per gallon. At $3.50 and 28 mpg that is about 12.5 cents a mile.
How much does it cost to charge an electric car?
Multiply the kilowatt-hours the car uses per mile by the miles driven and by your electricity price, adding the energy lost while charging.
Are electric cars cheaper to run than gasoline cars?
Often, but it depends on your electricity rate, your gasoline price, how you charge and how you drive. The EV vs gas calculator compares them on your numbers.
Why is my mileage lower than the sticker?
The sticker is a standardized test. Cold weather, speed, short trips, heavy loads and tire pressure all lower real mileage.
What is miles per gallon equivalent?
A figure that expresses an electric car's efficiency in the units of a gasoline car, based on the energy in a gallon of gasoline. It lets you compare the two, though cost per mile is the better comparison.
How do I compare a hybrid with an electric car?
Compute the cost per mile for each, using the fuel price for the hybrid and the electricity price for the electric car, and add purchase price, incentives and maintenance to compare total cost.
Which input moves the miles per gallon most in the MPG calculator?
Ranked by how far each moves the miles per gallon across the range tested: gallons of fuel used (6.8, 24%) and miles driven (5.6, 20%). Fuel price per gallon does not change it.
If I double gallons of fuel used in the MPG calculator, does the miles per gallon double?
Doubling it from 12.5 to 25 takes the miles per gallon from 28 to 14, which is 0.50 times the worked-example figure. So it falls instead of rising. Halving it to 6.3 gives 56.