How To Calculate Gas Mileage

Gas mileage, also called fuel economy, is the distance a vehicle travels on a given amount of fuel, and it can be quoted in two ways that look similar but behave in opposite directions. Litres per 100 kilometres states how much fuel a fixed distance demands, so 8.0 is a good figure and 12.0 is a poor one. Miles per gallon states how far a fixed volume carries you, so 29.401822916666664 is modest and 50 is excellent. The same car produces both numbers at once: 450 kilometres on 36 litres is 8.0 litres per 100 kilometres, which is exactly 29.401822916666664 miles per US gallon. Neither figure is more truthful than the other, because they are two views of one measurement.

The two scales are reciprocals, and that single fact explains almost every confusion on the subject. A reciprocal means that as one number rises the other falls, and by a proportional rather than a constant amount. Improve efficiency and litres per 100 kilometres drops while miles per gallon climbs, which is why the words better and worse swap direction between them. Halving consumption from 8.0 to 4.0 litres per 100 kilometres roughly doubles the miles per gallon from about 29.40 to about 58.80, but a fixed saving of one litre per 100 kilometres does not translate into a fixed gain in miles per gallon. Reading the two scales without noticing the inversion is the most common error in the topic.

Converting between the scales is not a single multiplication factor, because two unit conversions are stacked inside it. A mile is 1.609344 kilometres and a US gallon is 3.785411784 litres, so the conversion is (litres per gallon 3.785411784) divided by (kilometres per mile 1.609344), then multiplied by 100. Those two constants collapse into roughly 235.214583, which converts either way: divide it by miles per gallon to obtain litres per 100 kilometres, or divide it by litres per 100 kilometres to obtain miles per gallon. Applied to the worked case, 100 / 8.0 x 3.785411784 / 1.609344 gives 29.401822916666664 miles per gallon. Because the relationship is reciprocal, one constant serves both directions without any reversal.

The cost side uses the same litres and simply adds the pump price. Multiply the litres burned by the price per litre, then divide by the distance: 36 litres at 1.65 each costs 59.40, and dividing by 450 kilometres gives 0.132 per kilometre. Multiplying that rate by 100 gives 13.200000000000001 per 100 kilometres, which is the form most people find easier to compare. The cost per kilometre is the reusable figure, because it stays fixed as long as consumption and price do and simply multiplies by whatever distance you plan. Cost per kilometre moves with the L/100km figure rather than with mpg, since both are inverse measures, so a lower consumption number and a lower cost number always travel together.

A gallon is not one thing, and that is why miles per gallon is quoted differently in different countries. The US gallon is 3.785411784 litres exactly, while the imperial gallon used in the United Kingdom and a few other places is 4.54609 litres, about 20 per cent larger. A car that returns 29.401822916666664 miles per US gallon therefore returns roughly 35.3 miles per imperial gallon on the very same tank, without anything having changed about the vehicle. The fuel figure itself is identical; only the volume the word gallon denotes has changed. Whenever you read a miles-per-gallon number, the country of origin is part of the unit, not an incidental detail.

Because the two scales are reciprocal, a fixed saving matters very differently at each end of the range. Cutting consumption from 8.0 to 7.0 litres per 100 kilometres is a change of one litre, and it lifts the figure from about 29.40 to about 33.60 miles per gallon, a gain of roughly 4.2. Making the same one-litre cut from 4.0 to 3.0 litres per 100 kilometres lifts the figure from about 58.80 to about 78.40 miles per gallon, a gain of roughly 19.6. The absolute improvement is identical in litres per 100 kilometres, yet it is worth nearly five times as much on the miles-per-gallon scale. This is the arithmetic behind the observation that the last steps towards efficiency look dramatic in miles per gallon and modest in litres per 100 kilometres.

Measuring your own mileage properly takes two fills rather than one. Fill the tank to the same point each time, reset the trip odometer or record the reading, and note the litres added at the next fill, because those litres are what the intervening distance consumed. Dividing litres by distance and multiplying by 100 gives the consumption for that interval. A partial fill breaks the method, since the litres added no longer correspond to the distance covered, and a single fill cannot produce a figure at all. Averaging several intervals smooths out the variation caused by one long motorway run or one week of short cold trips.

Real consumption rarely matches the figure printed in a brochure, because that figure comes from a standardised laboratory cycle. Carrying passengers and luggage raises the mass the engine must move, air conditioning draws power that ultimately comes from fuel, and under-inflated tyres increase rolling resistance. Stop-start town traffic is worse than a steady cruise, because every acceleration wastes energy that braking then discards, and at high speed aerodynamic drag rises with the square of velocity. It is normal for real-world consumption to exceed the rated figure by ten to thirty per cent, and a loaded car on a hilly route can do considerably worse. Treating the rated number as a target rather than an ideal is the first step towards an accurate budget.

The practical value of all this is comparison, and the reusable quantity is cost per kilometre rather than either consumption scale. At 0.132 per kilometre, a 30 kilometre commute costs about 3.96 in fuel and the 450 kilometre trip costs 59.40, simply by multiplying the same rate. That figure also lets you weigh driving against a train fare or a taxi on equal terms, because those alternatives are usually priced per journey or per kilometre. When you do compare two cars, convert both to the same scale first, since a headline 8.0 litres per 100 kilometres and a headline 40 miles per gallon describe different efficiencies and cannot be subtracted as they stand. Keeping the units attached to the numbers as you work is the cheapest way to avoid the inversion.