Miles per gallon:
work out your real fuel economy

The formula and its inverse

If you drove 300 miles and the tank took 10 US gallons to refill, your car returned 30 miles per gallon. That is the whole idea: how far you travelled for each gallon of fuel it swallowed. A higher number means a thriftier car.

The figure is useful well beyond curiosity. It tells you what a road trip will cost, whether a service or a new set of tyres changed anything, and how a second-hand car compares with the claim in its brochure. The dashboard average is a convenient guess, but the pump and the trip meter do not flatter you.

To get a number worth keeping you need two readings taken the same way: miles travelled between two fill-ups, and the fuel volume the pump delivered at the second fill-up. Everything else is arithmetic.

Turn it round and you get the cost-side figure: gallons per mile, or gallons per 100 miles. A car doing 30 mpg uses 100 ÷ 30 = 3.33 gallons every 100 miles. Many countries quote the inverse form, litres per 100 km, because it makes savings easier to see: going from 10 to 8 L/100 km saves the same fuel no matter how far you drive.

The conversion between the two is a fixed constant. For US gallons, L/100 km = 235.215 ÷ mpg. For imperial gallons the constant is 282.481 ÷ mpg.

The full-tank method, step by step

1. 1Fill the tank until the pump clicks off, and reset the trip meter to zero (or note the odometer).
2. 2Drive as you normally would until the tank is at least half empty; a longer interval averages out stop-start noise.
3. 3Fill up again until the pump clicks off, the same way and at the same pace as before.
4. 4Write down the miles on the trip meter and the gallons the pump shows.
5. 5Divide miles by gallons.

Note that the fuel quantity you divide by is what you put back in, not the tank size. The tank is only refilled to the same level, so the fuel added equals the fuel burned.

US gallons versus UK gallons

Two different gallons are in use. A US gallon is 3.785 litres. An imperial (UK) gallon is 4.546 litres, about 20% larger. Because mpg divides by gallons, the larger unit makes the same car look better on paper.

So a UK car advertised at 45 mpg is roughly 37.5 mpg by US reckoning. Always check which gallon a figure uses before comparing a US-market car with a UK one. A converter is the safest way to translate between them.

Why your number moves from tank to tank

A single tank is a noisy measurement. Pumps stop at slightly different points, so a variation of half a gallon on a 10-gallon tank changes the answer by 5%. Averaging three or four fill-ups gives a steadier figure.

• Speed: drag rises with the square of speed, so motorway cruising at 70 mph costs noticeably more fuel per mile than 55 mph.
• Short trips: a cold engine runs rich, so frequent five-minute drives lower the average.
• Load and roof boxes: extra weight and aerodynamic drag both cost fuel.
• Tyre pressure: under-inflated tyres increase rolling resistance.
• Weather and fuel blend: winter fuel and cold air change both consumption and the miles you get.

Dashboard readout versus the pump

The average shown on the instrument cluster is calculated from the fuel injectors' flow and the speed sensor, not from what the pump actually delivered. It is useful for trends but tends to be optimistic, often by 2% to 5%, and a different tyre size or a small calibration error shifts it further.

A quick audit is to take three consecutive fill-ups, add the miles and add the gallons, and divide the totals. Summing before dividing, rather than averaging the three individual mpg figures, weights each tank by how far it went and gives the honest answer. Compare that with the dashboard average for the same period; if the dashboard consistently reads higher, you know by how much to discount it.

When the fill-up method misleads

The technique assumes the tank is brought back to the same level each time. If you stop at the first click on one occasion and top up generously on another, the fuel quantity no longer matches the fuel burned and the figure for those two tanks will swing in opposite directions.

• Pumps on a slope or with different nozzle shut-off behaviour can leave different amounts of room.
• Fuel that sloshes into the filler neck after a brisk stop makes a quick second click possible.
• Hybrids and plug-in cars add electricity, so petrol-only mpg overstates total efficiency; use a combined figure such as miles per gallon equivalent or consumption per kilowatt-hour.
• Towing, a heavy load or one long motorway run can make a single tank unrepresentative of your normal driving.

None of these problems is serious if you use several tanks. The aim is a repeatable number you can compare month to month.

Habits that change the number most

Of everything a driver controls, speed and acceleration matter most. Rolling up to a red light rather than racing to it, anticipating traffic and keeping a steady throttle commonly improve economy by 10% or more in town. On a motorway, dropping from 80 mph to 65 mph cuts aerodynamic drag substantially, because drag force grows with the square of speed and the power needed grows with its cube.

Maintenance helps in quieter ways: correct tyre pressure, clean air filters, the right grade of oil and wheel alignment each contribute a small share. Removing a roof rack when it is not in use is one of the cheapest fixes. Idling gets you zero miles per gallon, so switching off during a long wait saves fuel, though modern start-stop systems already do this for you.

Turning mpg into a trip cost

Once you know the number, the cost of any trip follows from a single line: gallons needed = distance ÷ mpg, and cost = gallons × price per gallon. A 240-mile journey at 30 mpg needs 8 gallons; at 25 mpg it needs 9.6 gallons, 1.6 gallons more.

The same logic works for comparing two cars. Because mpg is a rate per gallon, the difference between 20 and 25 mpg saves far more fuel than the difference between 40 and 45, even though the step looks identical. Over 10,000 miles, 20 → 25 mpg saves 100 gallons, while 40 → 45 saves only 27.8. That is why many agencies prefer gallons per 100 miles when ranking vehicles.