How to Convert Litres per 100km to MPG: US vs UK Gallons Explained

If you want a straight answer on how to convert litres per 100km to mpg, start with the maths, not a guess. This page works through the formula step by step, runs a worked example with the assumptions stated in full, and gives you a table you can read your own numbers off. Everything here uses the same definitions throughout, so the figures in the table match the figures in the example.

The short version is that how to convert litres per 100km to mpg comes down to two measurements you can take yourself: how far you drove, and how much fuel you used. Get those two right and the rest is arithmetic. Get either one wrong — a half-full tank, a trip meter that was never reset, a gallon confused with a litre — and the answer is meaningless, however carefully you divide.

How the MPG to L/100km conversion works — how to convert litres per 100km to mpg

MPG and litres per 100 km measure the same thing in opposite directions. Read the figures on this page about how to convert litres per 100km to mpg as a worked illustration of the constants, not as a figure for any particular car. MPG says how far you go on a fixed amount of fuel; L/100km says how much fuel you use over a fixed distance. To convert, you need the size of the gallon, and that is where most mistakes happen: a US gallon is 3.785 litres, while a UK imperial gallon is 4.546 litres, so the same car has a higher MPG figure in the UK than in the US. The conversion constants are 235.21 for a US gallon and 282.48 for an imperial gallon, so L/100km = 235.21 ÷ MPG (US) or 282.48 ÷ MPG (UK imperial). Going the other way, MPG (US) = 235.21 ÷ L/100km and MPG (UK) = 282.48 ÷ L/100km. Always label which gallon you mean, because a figure without that label cannot be checked. A car doing 40 MPG in the US does about 5.88 L/100km, while the same 40 MPG measured in imperial gallons is about 7.06 L/100km — the same driving, two different-looking numbers, purely because the gallon changed.

Worked example: converting a real figure — how to convert litres per 100km to mpg

Take a car that does 40 MPG. Read the figures on this page about how to convert litres per 100km to mpg as a worked illustration of the constants, not as a figure for any particular car. Using a US gallon, its consumption is 235.21 ÷ 40 = 5.88 L/100km. Using a UK imperial gallon, the same 40 MPG converts to 282.48 ÷ 40 = 7.06 L/100km. Both are correct; they describe the same fuel use with different gallons. Now go the other way: a car using 6.5 L/100km does 235.21 ÷ 6.5 = 36.2 MPG in US gallons, or 282.48 ÷ 6.5 = 43.5 MPG in imperial gallons. The common mistake is to convert with the wrong constant and then conclude the car is far more or less efficient than it really is. If you are comparing a UK figure with a US figure, convert both to L/100km first — it is the only unit that means the same thing on both sides of the Atlantic — and only then compare. The table below does that conversion for the most common MPG values, with both gallon types shown so the difference is visible at a glance.

MPG to L/100km conversion table — how to convert litres per 100km to mpg

Use the US column if your gallons are US gallons and the UK column if they are imperial gallons. Read the figures on this page about how to convert litres per 100km to mpg as a worked illustration of the constants, not as a figure for any particular car. The two columns describe the same fuel use; the UK figure is always higher because the imperial gallon is larger.

MPG L/100km (US gallon) L/100km (UK imperial gallon)
20 11.76 14.12
25 9.41 11.30
30 7.84 9.42
35 6.72 8.07
40 5.88 7.06
45 5.23 6.28
50 4.70 5.65

Why the UK figure is always higher — how to convert litres per 100km to mpg

The UK imperial gallon holds 4.546 litres; the US gallon holds 3.785 litres. For how to convert litres per 100km to mpg, the same reasoning applies — use the right gallon constant and the conversion stays consistent. That is a difference of about 20 percent, and it flows straight into MPG: the same distance on the same fuel gives a UK MPG figure about 20 percent higher than the US figure, because each imperial gallon goes further. Neither figure is wrong. They are the same economy expressed in different units, like miles and kilometres. The confusion starts when a UK figure is quoted next to a US figure without saying which gallon applies, or when a converter uses the wrong constant. If a car's economy looks surprisingly good or surprisingly bad, check the gallon before doubting the car. And when you compare cars from different markets, convert everything to L/100km first — it is the one unit that does not depend on which gallon you started with.

Common mistakes — how to convert litres per 100km to mpg

Five mistakes cover most wrong MPG figures. For how to convert litres per 100km to mpg, the same reasoning applies — use the right gallon constant and the conversion stays consistent. Mixing US and imperial gallons is the biggest, because it shifts the answer by about 20 percent with no warning. Using one tank instead of an average is next — a single fill is noisy, and a pump that clicks off early can add a phantom gallon. Forgetting to reset the trip meter at the fill is another, because it silently adds miles from the previous tank. Using the fuel light or the gauge instead of the pump reading is worse still, since gauges are approximate by design. Finally, comparing figures measured different ways — a laboratory test figure against a real-world figure, or a dashboard estimate against a brim-to-brim measurement — tells you nothing about the car and everything about the methods. Keep the method constant and the comparisons become meaningful.

Related calculations — how to convert litres per 100km to mpg

If this page answered your question, these pages take the next step — the same method applied to a closely related question, or the calculator that does the arithmetic for you. Read the figures on this page about how to convert litres per 100km to mpg as a worked illustration of the constants, not as a figure for any particular car.

Putting your how to convert litres per 100km to mpg figure to work

Putting the figure to work is where the method pays off. Read the figures on this page about how to convert litres per 100km to mpg as a worked illustration of the constants, not as a figure for any particular car. Once you know your own MPG, measured brim-to-brim over several tanks, you can price any journey before you set off: multiply the miles by the fuel price divided by your MPG, and the answer is the fuel cost of that trip at that example price. You can also compare cars honestly, because you are comparing measured figures rather than brochure claims. Keep a simple log — date, miles, gallons, price — and review it monthly. Patterns show up quickly: a winter dip, a motorway improvement, a slow decline that points to tyre pressures or a change in routes. The log turns a vague feeling about fuel use into a number you can act on, and that is the whole point of measuring in the first place.

Frequently asked questions

How do I convert MPG to L/100km?

Divide 235.21 by the MPG for a US gallon, or 282.48 by the MPG for a UK imperial gallon. That is the point to carry into how to convert litres per 100km to mpg: label the gallon every time, and the two systems stop disagreeing with each other. So 40 MPG is 5.88 L/100km (US) or 7.06 L/100km (UK). Label which gallon you used, because the two answers differ.

Why is UK MPG higher than US MPG for the same car?

Because the UK imperial gallon is larger than the US gallon — 4.546 litres against 3.785 litres. Read the figures on this page about how to convert litres per 100km to mpg as a worked illustration of the constants, not as a figure for any particular car. The same distance on the same fuel gives a UK MPG figure about 20 percent higher. Neither figure is wrong; they use different gallons.

How do I convert L/100km back to MPG?

Divide the constant by the L/100km figure: MPG (US) = 235.21 ÷ L/100km, and MPG (UK) = 282.48 ÷ L/100km. For how to convert litres per 100km to mpg, the same reasoning applies — use the right gallon constant and the conversion stays consistent. A car using 6.0 L/100km does 39.2 MPG (US) or 47.1 MPG (UK).

Which is better, a higher MPG or a lower L/100km?

Both mean the same thing: higher MPG is better, and lower L/100km is better. That is the point to carry into how to convert litres per 100km to mpg: label the gallon every time, and the two systems stop disagreeing with each other. The two scales run in opposite directions, which is why the conversion is a division rather than a multiplication, and why it is easy to read a converted figure backwards if you are not careful.

For how to convert litres per 100km to mpg, can I compare a UK MPG figure with a US MPG figure directly?

No — not until both are in the same units. That is the point to carry into how to convert litres per 100km to mpg: label the gallon every time, and the two systems stop disagreeing with each other. Convert both to L/100km first, using the right constant for each gallon, and then compare. Comparing the raw MPG figures directly will make the UK car look about 20 percent more efficient than it really is relative to the US car.