How to Calculate Your Real Fuel Economy by Hand

Because the trip computer is an estimate built from the engine's fuel injector timings and a distance signal, and it is commonly 3 to 8% optimistic. The pump is a calibrated instrument checked by trading standards. Two receipts and an odometer give you a figure you can defend, which matters if you are deciding whether a car is faulty, comparing two cars you own, or checking a seller's claim.

It also catches problems. A steady decline in measured economy over a few months is one of the earliest signs of a tyre losing pressure, a sticking brake, a failing oxygen sensor or a clogged filter. The computer's figure, being an estimate, can drift without telling you anything.

Fill the tank until the pump shuts off the first time, note the odometer or reset the trip meter, drive as you normally would until the tank is at least half empty, then fill again in the same way and keep the receipt. The distance driven divided by the fuel added is your economy. That is the whole method; everything else is about doing it consistently.

The first click matters because pumps shut off at slightly different levels depending on the nozzle, the angle of the car and the temperature of the fuel. Stopping at the first click every time, rather than topping up, makes each fill comparable with the last. Using the same pump at the same station removes another variable.

The half-tank minimum is about resolution. If you add 20 litres and the pump's shut-off point varies by half a litre, the error is 2.5%. If you add 45 litres, it is closer to 1%. Longer samples are better samples, and averaging three or four fills is better still.

MPG = miles driven ÷ gallons added. If the pump gave litres, convert first: divide litres by 3.785411784 for US gallons or by 4.54609 for imperial gallons. A British driver who covered 412 miles and added 41.6 litres has used 9.15 imperial gallons, and 412 divided by 9.15 is 45.0 mpg (UK). The litres to gallons converter does the first step.

An American driver who covered 380 miles on 12.3 US gallons gets 30.9 mpg (US) with no conversion needed. If that driver wants to compare with a British friend's figure, multiply by 1.200950 to get 37.1 mpg (UK), or use the US to UK mpg page. The gallon has to match before the figures mean the same thing.

L/100km = litres added ÷ kilometres driven × 100. A driver who covered 645 km and added 41.6 litres has used 6.45 L/100km. If the odometer reads in miles, convert with the miles to km page first: 412 miles is 663 km, and 41.6 litres over 663 km is 6.27 L/100km.

For km/L, divide the other way: kilometres driven divided by litres added. 645 km on 41.6 litres is 15.5 km/L. The two metric units are reciprocals, so 100 divided by 6.45 gives the same 15.5. Whichever you compute, the L/100km to MPG and km/L to MPG pages turn it into a gallon-based figure if you need one.

Monday: fill to first click, odometer 48,213 miles, trip reset. The following Saturday: fill to first click at the same pump, trip reads 397.4 miles, receipt says 38.92 litres. The car is British, so the gallon is imperial: 38.92 divided by 4.54609 is 8.56 gallons. 397.4 divided by 8.56 is 46.4 mpg (UK). In US terms, 46.4 divided by 1.200950 is 38.7 mpg. In metric, 397.4 miles is 639.6 km, and 38.92 litres over 639.6 km times 100 is 6.09 L/100km.

Three units, one fill, and they all agree because they are the same measurement expressed three ways. The converter on any page of this site will confirm the cross-check: 46.4 mpg (UK) is 6.09 L/100km.

Write down three things at every fill: the date, the odometer reading and the litres or gallons added. Nothing else is needed. A note on your phone works; a spreadsheet with those three columns and a fourth for the calculated figure works better, because it will show the trend. A calculator that takes miles and fuel in any unit does the arithmetic for each row.

Keep the units the same in every row. A log that switches between litres and gallons, or between miles and kilometres, is a log that will be misread. If your car came from a country with a different convention from yours, pick one and convert everything to it on the day you write it down, not later.

A steady figure over months is the normal picture. A sudden drop of 10% or more with no change in your driving is worth investigating: tyre pressures first, then anything the car has started doing differently.

• Topping up past the first click on one fill but not the other. This alone can swing a single-tank figure by 5%.
• A very short sample. A quarter of a tank is noise.
• A different pump or a different station, which changes the shut-off level.
• Forgetting which gallon the pump used, when the pump is in gallons. US and imperial differ by 20%.
• Mixing miles and kilometres between the odometer and the calculation.
• Treating one tank as the answer. Weather, traffic and one long motorway run all move a single tank by 10% or more. Average several.