US and imperial gallons must be distinguished
One US liquid gallon is 3.785411784 L; one imperial gallon is 4.54609 L. Thus 30 US mpg is about 7.84 L/100 km, while 30 imperial mpg is about 9.42 L/100 km. The same vehicle would have a numerically larger imperial mpg value, so comparing two unlabelled mpg figures is unsafe.
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. Full-tank fuel economy is distance traveled divided by fuel added to restore the same fill level. 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.
Compare fuel and electricity over the same miles
Assume 10,000 miles, a gasoline vehicle at 30 US mpg and gasoline $3.50/gallon. Fuel cost is about $1,166.67. For a separate assumed EV wall-energy requirement of 0.30 kWh/mile and tariff $0.20/kWh, energy cost is $600.
Difference is $566.67 before other ownership costs. EPA wall-energy figures already include charging losses; adding them again double-counts that allowance. Battery-side energy needs a separately stated charging-efficiency assumption.
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. Charging loss depends on the vehicle, charger, power level and conditions.
Use measured wall energy or a stated scenario loss, and do not add a second loss allowance to a wall-energy figure. 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 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. An annual fuel-cost estimate depends on achieved mileage under the driving conditions being modeled.
A best-case rating can understate fuel use in other conditions.
Assumed inputs and calculated outputs
MPG Calculator: input sensitivity
| 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 |
What goes wrong
Comparing fuel economy: mpg, L/100 km and electric cars: common errors
| 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. |
Comparing fuel economy: mpg, L/100 km and electric cars: worked calculation
Using the MPG Calculator worked example, moving Miles driven from 280 to 420 changes the Miles per gallon from 22.4 to 33.6.
| Input | Value | Definition |
|---|---|---|
| Miles driven | 350 | Entered example assumption. |
| Gallons of fuel used | 12.5 | Gallons added at the fill-up that follows the miles driven. |
| Fuel price per gallon | $3.50 | Entered example assumption. |
| Miles per gallon | 28 |
|---|---|
| Fuel cost per mile | $0.13 |
| Liters per 100 km | 8.4 |
| 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 |
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.
Comparing fuel economy: mpg, L/100 km and electric cars: points to carry forward
- MPG and L/100 km vary inversely; US and imperial gallon conventions produce different MPG values.
- Fuel used per distance provides an additive cost basis. Miles per gallon is its reciprocal, so averaging MPG directly can distort a combined trip.
- Several full-tank observations reduce the influence of one fill's shutoff point and a single trip's conditions. Total miles divided by total gallons gives the combined MPG.
- For a gasoline vehicle, cost per mile is fuel price divided by mpg. For fixed electric consumption, a hypothetical 10% increase in electricity price raises modeled energy cost per mile by 10%; it does not change the energy needed for that distance.
- Add charging losses and use the electricity rate you will really pay.
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?
Miles per gallon equals distance traveled between full-tank readings divided by the refill gallons. 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.