Why Your Real MPG Differs From the Official Figure

Updated 8 November 2026. Every figure on this page comes from the same formulas as our calculators.

Almost every driver finds that their car does not quite match its official fuel economy. That does not mean the figure is false: it was measured in a way that makes cars comparable, not in the way you drive. Here is what makes the difference and how to measure your own figure properly.

How official figures are measured

Official figures come from laboratory tests on a rolling road, with a set speed pattern, temperature and procedure. Every car of that type goes through the same test, which is what makes the figures useful for comparing cars. The US EPA adjusts its results to be closer to real-world driving; the European WLTP test is more realistic than the older NEDC test it replaced. See US MPG, UK MPG and L/100 km for the differences between the tests.

What changes your real figure

Speed: air resistance grows with the square of speed, so motorway and highway driving at high speed uses noticeably more fuel per mile.
Short trips and cold starts: an engine uses more fuel until it warms up, so many short journeys pull the average down, especially in winter.
Temperature and weather: cold air, wet roads, headwinds and winter fuel blends all cost economy.
Climate control: air conditioning and, in electric cars, heating draw extra energy.
Tire pressure and tires: under-inflated tires roll with more resistance. Off-road and winter tires usually use more fuel than the tires fitted for the official test.
Load and accessories: passengers, cargo, roof boxes and bike racks add weight and drag.
Driving style: hard acceleration and late braking turn fuel into heat in the brakes.
Plug-in hybrids: the official combined figure assumes regular charging. Driven mostly on petrol, a plug-in hybrid's real figure can be far lower.

Changed tire size? Your MPG reading is off too

If you fitted taller tires, the odometer under-counts the miles, so any MPG worked out from it looks worse than it really is. With a 5 % taller tire, 20 MPG on paper is about 21 MPG in reality. The bigger tires guide explains why, and the tire size and fuel economy calculator corrects the figure.

How to measure your real MPG: fill to fill

Fill the tank until the pump clicks off, and reset the trip meter.
Drive normally until the tank is at least half empty.
Fill up again until the pump clicks off, and note the amount of fuel added.
Divide the miles on the trip meter by the gallons added.

For example, 312.4 miles on 11.2 US gallons is 27.9 MPG, which is 33.5 UK mpg or 8.43 L/100 km. Try it in the MPG calculator.

One tank can be misleading, because the pump does not always click off at the same level. Average several fills for a reliable figure. The trip computer's own MPG display is an estimate and can differ from the pump figure.

Using official figures sensibly

Official figures are best for comparing cars against each other. For budgeting, use your measured figure in the trip fuel cost calculator; if you do not have one yet, expect to land somewhat below the official number.