Why is MPG a misleading way to compare cars?
MPG measures distance per fuel and L/100km measures fuel per distance, so they are reciprocals rather than a scale. That inversion makes equal MPG gains represent wildly unequal fuel savings, an effect known as the MPG illusion.
Updated 2026-08-24
The illusion, and why it changes what you should buy
This is the most useful thing on this page, and it is genuinely counterintuitive, so it is worth working through with numbers.
Consider two possible upgrades. A household replaces a vehicle doing 10 miles per gallon with one doing 20. Another replaces one doing 25 with one doing 50. The second looks far more impressive — a jump of 25 miles per gallon against a jump of 10 — and it saves less fuel.
Over 10,000 miles, the first pair consumes 1,000 gallons and then 500, a saving of 500 gallons. The second consumes 400 gallons and then 200, a saving of 200. The upgrade with the smaller apparent improvement saves two and a half times more fuel.
The reason is that miles per gallon is distance divided by fuel, so fuel consumed is distance divided by MPG. That is a reciprocal relationship, and reciprocals compress at the top end. Each additional mile per gallon saves less than the one before it, so the same numerical gain means completely different things depending on where it starts. Going from 10 to 11 MPG saves more fuel over a year than going from 33 to 50.
Richard Larrick and Jack Soll documented this in Science in 2008 and named it the MPG illusion, showing that people asked to rank fuel savings do so incorrectly and consistently, because they treat MPG as a linear scale. The finding is part of why the United States Environmental Protection Agency added gallons per 100 miles to fuel economy labels, and why most of the world expresses consumption as litres per 100 kilometres in the first place.
The practical rule is that consumption-per-distance ranks savings correctly and economy-per-fuel does not. If you are comparing options on running cost, convert to litres per 100 kilometres or gallons per 100 miles first, and the comparison becomes a straightforward subtraction.
The conversion, and the two different gallons
Because MPG and L/100km are reciprocals, the conversion is a division rather than a multiplication, and the constant depends on which gallon is meant.
For the United States gallon, MPG multiplied by L/100km equals 235.215. So dividing 235.215 by a figure in either unit gives the figure in the other. A car at 30 US MPG is 7.84 L/100km; a car at 8 L/100km is 29.4 US MPG. The symmetry means one constant covers both directions.
For the imperial gallon, used in the United Kingdom, the constant is 282.481. The imperial gallon is 4.546 litres against the United States gallon at 3.785, about twenty percent larger, so the same vehicle quotes roughly twenty percent more miles per gallon in British figures than in American ones. A car advertised at 40 MPG in the United Kingdom is about 33 MPG in the United States, and neither figure is wrong.
This is a real source of confusion in car reviews and forum posts, where a figure is quoted without a country and the reader assumes their own. The gap is large enough to change a purchasing decision. Anything quoting MPG without saying which gallon is ambiguous by roughly a fifth.
Electric vehicles sidestep the whole problem by using energy per distance directly — kilowatt hours per 100 kilometres, or miles per kilowatt hour in the United States, which reintroduces exactly the same reciprocal issue. The MPGe figure on American labels converts electricity into a gasoline-equivalent using 33.7 kilowatt hours per gallon, which makes cross-fuel comparison possible and inherits the illusion along with the unit.
Speed conversions worth knowing
These are simpler than fuel economy because they are genuinely linear, and a few factors are worth holding in memory.
A mile is exactly 1.609344 kilometres, fixed by international agreement in 1959. So miles per hour multiplied by 1.609344 gives kilometres per hour, and the reverse divides. For mental arithmetic, multiplying by 1.6 is close enough for anything on a road sign, and a useful trick is that consecutive Fibonacci numbers approximate the ratio: 50 miles is about 80 kilometres, 80 miles about 130.
The common limits are worth memorising directly. 30 mph is 48 km/h, 60 mph is 97 km/h, 70 mph is 113 km/h. Going the other way, 50 km/h is 31 mph, 100 km/h is 62 mph, and 130 km/h is 81 mph. A driver crossing between systems who reads 100 as familiar is the origin of a great many speeding tickets.
The knot is the odd one and has a good reason to exist. It is one nautical mile per hour, and the nautical mile was defined as one minute of arc of latitude — so a vessel travelling one knot covers one minute of latitude in an hour. That makes navigation arithmetic on a chart trivial, which is why marine and aviation use has never moved to kilometres. The nautical mile is now fixed at exactly 1,852 metres, so a knot is 1.852 km/h or about 1.151 mph.
The pattern across all of these is that speed converts by a constant and consumption does not. Anything expressed as a ratio where the quantity of interest is in the denominator behaves like MPG, and that includes fuel economy, price per unit and any rate expressed as amount-per-thing rather than thing-per-amount.
What a journey actually costs
Fuel cost for a trip is distance divided by economy, multiplied by fuel price, with attention to units at each step. In metric terms it is distance in kilometres, times consumption in litres per hundred kilometres, divided by a hundred, times price per litre.
Fuel is not the whole cost, and for a decision about whether to drive it is often not even most of it. Depreciation, tyres, servicing and the share of insurance attributable to mileage all scale with distance. The United States Internal Revenue Service standard mileage rate exists precisely to capture this — it is set annually to approximate the full per-mile cost of operating a vehicle, and it has run at several times the fuel cost alone. Reimbursement at fuel cost only is systematically under-paying the driver.
Real-world economy also differs from the quoted figure, generally downward. Published figures come from standardised test cycles, and although modern procedures are closer to reality than the ones they replaced, they still assume moderate acceleration, no roof box, correct tyre pressure and mild weather. Short trips are the worst case, because a cold engine consumes substantially more until it reaches operating temperature, so a pattern of brief journeys produces economy well below the label.
Speed matters more than most drivers expect, because aerodynamic drag rises with the square of speed and the power to overcome it with the cube. Most cars are meaningfully less efficient at 130 km/h than at 100, and the time saved on a long journey is smaller than it feels — an hour at 130 rather than 100 saves about fourteen minutes over a hundred kilometres, at a fuel penalty that frequently exceeds twenty percent.