Measuring it properly: the brim-to-brim method
The fairest way to measure your own fuel economy is the same one manufacturers used before laboratory cycles existed: brim to brim. Fill the tank completely, to the first automatic click of the pump nozzle, and note the odometer reading. Drive as you normally would. At the next stop, fill the tank completely again, to the same first click, and note the fuel added and the new odometer reading. That fuel amount is what it took to cover the distance between the two fill-ups, which is the fairest single-tank reading you can take without special equipment.
A single tank still lies a little, and it is worth knowing why before you trust one reading too far. The pump's automatic shut-off does not click at exactly the same fill level every time; forecourt slope and the angle the nozzle sits at shift it slightly, and a warm tank expands the fuel already inside it, changing how much more fits. None of these effects is large on its own, but together they can move a single reading by a few per cent in either direction, enough to make one tank look meaningfully better or worse than your car really is. Because those small errors do not consistently favour the same side, averaging three or more fill-ups cancels most of them out and lands much closer to your car's true figure than any one reading, especially if driving conditions varied between fill-ups.
The sum behind the number
Once you have a distance and a fuel amount for the same stretch of driving, the arithmetic is short. Miles per gallon divides distance by fuel; the metric measures just flip which one leads.
Miles per gallon comes in two sizes because a gallon does. The UK (imperial) gallon is exactly 4.54609 litres; the US gallon is exactly 3.785411784 litres, a noticeably smaller volume. Put the same 40 litres through the same 350-mile drive and you get roughly 39.78 mpg by the UK measure but roughly 33.12 mpg by the US one, a real difference of nearly seven mpg that has nothing to do with the car and everything to do with which gallon is being counted. Always check which mpg a figure you are reading uses before comparing it with your own.
Why your figure differs from the brochure
New cars sold in the UK and the EU are rated under WLTP, a laboratory cycle that replaced the older, more optimistic NEDC test for all new registrations by September 2018. WLTP runs longer and faster than NEDC did and mixes in more motorway-style driving, so it comes closer to real conditions than its predecessor. In the US, the EPA's window-sticker figure already carries a downward adjustment intended to model everyday driving rather than a raw lab result. Even so, both remain controlled tests: a fixed temperature, no wind, gentle scripted acceleration, and no air conditioning or heavy load. Ordinary driving brings all of those back, plus hills, traffic, a cold start on a short trip, and however heavily your right foot happens to sit that day, which is why real-world economy commonly lands below the official figure whichever test produced it. Treat any brochure number as a best case rather than a target, and trust your own brim-to-brim measurement for what your car is actually doing.
mpg, L/100 km and km/L: three ways to read the same tank
All three measures describe one fill-up, but two of them run in the opposite direction from the third, and mixing that up is an easy mistake. Miles per gallon and kilometres per litre both rise as economy improves, because both count how far a fixed amount of fuel carries you: a bigger number is always the better car. Litres per 100 km falls as economy improves instead, because it counts the opposite thing, how much fuel a fixed distance costs: a smaller number is the better car there. That is exactly why L/100 km suits budgeting a long trip, where the question is how many litres it will take, while mpg suits the older, more familiar question of how far a tank goes.
A short conversion table, working from mpg (UK), helps the three mental models line up:
| mpg (UK) | mpg (US) | L/100 km | km/L |
|---|---|---|---|
| 30 | 25.0 | 9.42 | 10.62 |
| 40 | 33.3 | 7.06 | 14.16 |
| 50 | 41.6 | 5.65 | 17.70 |
| 60 | 50.0 | 4.71 | 21.24 |
Questions people ask
What is the brim-to-brim method?
Fill the tank fully, note the odometer, drive as normal, then fill fully again at the next stop and note the new odometer reading and how much fuel went in. That fuel amount is what it took to cover the distance between the two fill-ups, which is the fairest single-tank measurement you can take without special equipment.
Why does my mpg never match the brochure figure?
Brochure figures come from a laboratory cycle, WLTP in the UK and EU or an EPA-derived figure in the US, run at a fixed temperature with no wind, hills, air conditioning or heavy load. Ordinary driving includes all of those, so real-world economy commonly comes in below the label. Treat the brochure number as a best case, not a promise.
Is mpg (UK) the same as mpg (US)?
No. Both count miles per gallon, but the UK (imperial) gallon is 4.54609 litres and the US gallon is a smaller 3.785411784 litres. The same car and the same fill-up produce a lower US mpg figure than a UK mpg figure, roughly by a factor of 0.83, purely because the "gallon" is a different size, not because the car used more fuel.
Should I judge my mileage on one tank or an average?
An average over several fill-ups. A single tank's fuel amount depends partly on the exact point the pump's nozzle clicks off and how full "full" really was, so one reading can be off by a few per cent either way. Those small errors do not consistently favour the same side, so averaging three or more fill-ups cancels most of them out and gets you much closer to your car's true figure.
What counts as a good mpg for my car?
It depends heavily on the car, so treat these as wide, typical bands rather than fixed targets: larger SUVs and pick-ups commonly sit somewhere in the 20s to mid-30s mpg (UK), an average family petrol or diesel car typically runs from the mid-30s to the mid-50s, and the most efficient small cars and hybrids often clear 55 to 60 mpg or more. Engine size, weight, gearing and how you drive all move a car within or outside its usual range.
Why did my mpg suddenly drop this tank?
Short trips are the most common cause: a cold engine is thirstier before it reaches operating temperature, and on a short journey a large share of the fuel goes towards that warm-up rather than the driving itself. Cold weather, underinflated tyres, a heavier load, more stop-start town driving, or simply a different mix of roads can all pull the figure down too, usually several at once rather than any single cause.
How accurate is the full-tank range estimate?
Only as accurate as the economy it is built from, and it assumes the rest of the tank gets driven the same way as the fill-up you measured. A longer motorway run afterwards will usually go further than the estimate; a lot of short, cold, urban driving will usually fall short of it. Treat it as a planning guide rather than a guarantee, and keep a safety margin rather than running to the number exactly.
This calculator works out fuel economy from the distance and fuel amount entered; it cannot know your car's condition, the fuel actually used, or how representative this particular tank was of your normal driving. Reference bands and range estimates are broad guides, not a substitute for tracking several fill-ups over time.