Gas mileage:
how to work out your real miles per gallon

What gas mileage actually measures

Gas mileage tells you how far a vehicle travels on a fixed amount of fuel. In the United States it is quoted as miles per US gallon (mpg). Most of the rest of the world flips the ratio and quotes litres used per 100 kilometres, while India and several other markets prefer kilometres per litre. All three describe the same thing: distance against fuel.

The figure you see on the instrument cluster is calculated from the injector signal and speed, and it can drift a few percent from the truth. The pump is the honest referee. Fuel added at a full-tank fill-up is measured by the dispenser, and the distance comes from the odometer, so the ratio of the two cannot be argued with.

It also helps to keep the direction of each scale in mind. Higher mpg and higher km/L are better. For L/100 km it is the opposite: a lower number means the car is thriftier. Mixing these up is behind many arguments about which car is more efficient.

The fill-up method and the formula behind it

Fill the tank to the automatic click-off, note the odometer, and reset the trip meter if you like. Drive normally. At the next fill-up, again stop at the first click-off and record how much fuel the pump delivered. That quantity is exactly what the car burned over the distance since the previous full tank.

Fuel economy in litres per 100 km uses the inverse idea: how much fuel does 100 km cost?

A calculator does these divisions instantly and converts between the three units, which is useful when a spec sheet is in one unit and your fuel receipts are in another.

A worked example from two receipts

Take a road trip logged with the full-tank method. The first fill-up showed 48,215 miles on the odometer. At the next fill-up the odometer read 48,527 miles and the pump delivered 11.6 US gallons.

To check the metric version: 312 miles is 502.1 km and 11.6 US gallons is 43.9 litres. Dividing 502.1 by 43.9 gives 11.4 km/L, and 100 ÷ 11.4 is 8.7 litres per 100 km. Both routes land on the same car.

Converting between mpg, km/L and L/100 km

Because L/100 km is an inverse of distance-per-fuel, you cannot convert by multiplying. The conversions go through a constant, 235.215, which comes from 100 km expressed in miles and one US gallon expressed in litres.

The same constant works backwards: mpg = 235.215 ÷ L/100 km. For km/L, remember that 1 km/L is roughly 2.352 US mpg.

Note how the gains shrink at higher mpg. Moving from 20 to 25 mpg saves 2.4 L per 100 km, while 38 to 50 saves only 1.5. That is why efficiency gains are easiest to see in L/100 km.

Why your figure moves from one tank to the next

Two tanks in a row rarely give the same answer, and that is normal. Fuel economy responds to the things you are doing, and the full-tank method records all of them at once.

• Short trips: a cold engine runs rich, so a week of 3 km errands scores noticeably worse than one long highway run.
• Speed: air resistance climbs with the square of speed, so cruising at 120 km/h costs far more per kilometre than 90 km/h.
• Load and tyres: low tyre pressure and a roof box both add drag; a few extra people add weight that costs fuel in town.
• Weather and fuel: cold air is denser, winter fuel blends differ and air-conditioning adds load in hot weather.
• Pump differences: click-off points vary a little between nozzles and slopes, which can move a single-tank result by 1 to 3 percent.

Averaging three or four tanks smooths most of this noise and is the number to quote when comparing cars or judging a service.

The gallon trap when comparing countries

One catch trips up people comparing a British review with an American one. The UK gallon is about 20 percent larger than the US gallon, so the same car shows a bigger mpg number in the UK. A car rated at 40 mpg in a British magazine corresponds to roughly 33 mpg on the US scale, because 40 × 0.8327 = 33.3.

Litres per 100 km sidesteps the problem. Both 40 imperial mpg and 33.3 US mpg come out at about 7.06 L/100 km, which is why engineers and most regulators outside North America prefer it.

Electric vehicles borrowed the mpg idea as MPGe, based on the energy content of a gallon of petrol, taken as 33.7 kWh in the US test method. It lets you compare a plug-in with a petrol car on one scale, though the cost per mile depends on local electricity and fuel prices rather than the energy figure alone.

Using your number for decisions

Once you have a trustworthy figure you can turn it into running cost. Fuel cost per distance is the price per unit of fuel divided by the distance you get from it, so a better mileage cuts the cost directly. Look up the current pump price in your area, because it changes often.

It also gives you an early warning system. A car that suddenly drops by more than 10 percent over several tanks while driving habits stay the same is worth a check: tyres, air filter, a dragging brake, or a failing sensor are all common causes.

When comparing against the manufacturer's rating, remember that official figures come from standardised laboratory cycles. Real driving often returns less, so use the rating to rank cars and your fill-up number to budget.