Two gallons, and why the page exists
The United States kept the Queen Anne wine gallon, defined as 231 cubic inches, which works out to exactly 3.785411784 liters. Britain replaced its assorted gallons in 1824 with the imperial gallon, originally the volume of ten pounds of water and now defined as exactly 4.54609 liters. They were never reconciled, and both are still in daily use for fuel economy in their respective countries.
The imperial gallon is 20.2 percent larger, so an imperial MPG figure is 20.2 percent higher than the US figure for the same car. That is not a rounding difference — it is the gap between a car that looks efficient and one that looks ordinary. A British magazine reporting 50 mpg is describing a car an American publication would call 41.6 mpg. Going the other way, a US window sticker showing 30 mpg would be 36.0 mpg in the UK press. If a number came off a foreign website and you did not check which gallon it used, it is probably wrong by a fifth.
Working the conversion by hand
Start from 30 MPG (US). Multiply by 1.609344 km per mile and divide by 3.785411784 liters per gallon: 30 x 1.609344 / 3.785411784 = 12.754 km/L. Invert and scale for the European measure: 100 / 12.754 = 7.84 L/100km. For imperial MPG, run the first conversion backward with the bigger gallon: 12.754 x 4.54609 / 1.609344 = 36.03 mpg. So one car is simultaneously 30 mpg, 36 mpg, 7.84 L/100km and 12.75 km/L, and every one of those numbers is correct.
All four constants above are exact by definition rather than measured, so the conversion has no inherent error. Any discrepancy against another calculator is rounding, or that calculator using 3.79 and 1.61.
The inverse-measure trap
MPG and km/L count distance per unit of fuel; L/100km and gallons per 100 miles count fuel per unit of distance. They are reciprocals, and that has a consequence people consistently get backward: equal steps in MPG are not equal savings.
Going from 15 to 20 MPG saves 1.67 gallons per 100 miles. Going from 30 to 35 MPG — the same five-MPG improvement — saves only 0.48 gallons per 100 miles, less than a third as much. Going from 40 to 50 saves 0.50 gallons. This is why replacing the worst vehicle in a fleet does far more than upgrading an already-efficient one, and it is the reason the EPA added a gallons-per-100-miles figure to the window sticker. The L/100km convention has this property built in: the number is directly proportional to fuel burned and to fuel cost, so you can compare and average the figures without thinking about it.
Comparing against an electric car
You cannot compare MPG to miles per kWh, because the units measure different substances. The only meaningful comparison is money per mile, which is what the cost section computes: gallons per 100 miles times the gas price against kWh per 100 miles times the electricity price.
The result swings wildly on the electricity price, which is why the break-even figure is the more useful output. A 30 MPG car at $3.50 a gallon costs $11.67 per 100 miles. A 3.5 mi/kWh EV covering the same 100 miles uses 28.6 kWh, which at a home rate of $0.15 costs $4.29 and at a DC fast charging rate of $0.50 costs $14.29 — more than the gas car. Whether an EV is cheaper to run is a question about where you charge it, not about the vehicles.
MPGe, which appears on US EV window stickers, is a separate thing: miles per kWh times 33.7, the energy in a gallon of gasoline. It answers "how much energy does this use" and is fine for comparing EVs against each other. It says nothing about cost, because a kWh and a gallon do not cost proportionally to their energy content.
Why your own number is lower than the sticker
EPA combined economy is 55 percent city and 45 percent highway, measured on a dynamometer under a defined schedule with adjustment factors applied. Real driving runs about 10 to 20 percent below it for most people, and the gap widens with short trips from cold starts, sustained speeds above 70, roof boxes, low tires and any significant use of heating or air conditioning. EVs lose more in winter than gas cars do, since resistive cabin heat is drawn from the same battery that moves the car and there is no waste engine heat to use — 20 to 40 percent off the rated range in freezing conditions is normal, not a fault.
The practical advice: for cost estimates, use your own trip computer average over a few tanks, not the sticker. For comparing two vehicles you have not bought, the sticker is the only figure measured the same way for both, and that is exactly what it is for.
Questions people ask
Is 30 MPG (US) really 36 MPG in the UK?
Yes — 36.0, to be precise. The imperial gallon is 4.54609 liters against the US gallon's 3.785411784, so it holds 20.2 percent more fuel and the same car travels 20.2 percent farther on one. The same figure is 7.84 L/100km and 12.75 km/L. Nothing about the car changed; the denominator did.
Which unit should I actually use?
Whichever your country prints on the pump and the sticker, for anything you are going to discuss with other people. For your own comparisons, the fuel-per-distance forms — L/100km or gallons per 100 miles — behave better arithmetically: they scale linearly with fuel used and with cost, so you can average them across a mixed fleet or a mixed trip, which you cannot correctly do with MPG.
Does the fuel type change the conversion?
No. The unit conversion is pure volume and distance arithmetic and is identical for gasoline, diesel and E85. What differs is what each fuel costs and how much energy it carries, so a diesel that returns better MPG is not automatically cheaper per mile when diesel is priced above gasoline. Enter the price you actually pay for the fuel you actually buy and compare cost per mile.
What is MPGe and should I compare it to MPG?
MPGe is miles per kilowatt-hour multiplied by 33.7, the EPA's figure for the energy in a gallon of gasoline. It puts EVs on a scale that looks like MPG so labels can be read side by side, and it is legitimate for comparing energy use. It is not a cost comparison and should not be treated as one: at typical residential rates a kWh is far cheaper per unit of energy than gasoline, so a 100 MPGe EV usually costs much less per mile than a 100 MPG car would, if such a car existed.
Why do I get a slightly different answer from another converter?
Rounded constants, almost always. The definitions are exact — 3.785411784 L per US gallon, 4.54609 L per imperial gallon, 1.609344 km per mile — and this tool uses all the digits. A converter using 3.79 and 1.61 will land within a tenth of an MPG, which is inside the noise of any real-world measurement but will not match to two decimal places.