How to calculate your car's real mpg and cost per mile
Work out your car's real fuel economy with the full-tank method, convert litres to gallons, and calculate cost per mile at today's UK fuel prices.
The most reliable way to find your car's real fuel economy is the full-tank method: fill the tank, note the mileage, drive as normal, fill up again and divide the miles by the fuel you put in. It takes one tank of fuel and a calculator, and it tells you far more than the figure in the brochure.
Once you know your mpg, cost per mile is one more step. A car doing 45 mpg on E10 petrol at 174.7p a litre, the UK average on 11 October 2026, costs about 17.6p a mile in fuel. That is around £1,765 for 10,000 miles.
The full-tank (brim-to-brim) method
The idea is simple: when you fill a tank that was full the last time, the fuel you put in is the fuel you used since. Here is how to do it.
- Fill the tank until the pump clicks off.
- Note the odometer reading, or reset a trip meter to zero.
- Drive as you normally would. A whole tank gives a more reliable figure than a few miles.
- Fill up again until the pump clicks off, ideally at the same station and pump.
- Note the litres you put in and the miles driven since the last fill.
- Divide the miles by the fuel used, converted to gallons, to get mpg.
Tips for a consistent fill
The method depends on the tank being equally full each time. A few habits help:
- Stop at the first click of the automatic cut-off rather than squeezing in extra.
- Use the same pump, or at least a level forecourt, so the car sits at the same angle.
- Keep your receipts, which show the exact litres and price.
- Repeat over several tanks and average them. Any single fill can be a litre or so out.
Converting litres to UK gallons
UK pumps sell fuel in litres, but UK economy is usually quoted in miles per gallon. One UK (imperial) gallon is exactly 4.54609 litres.
To convert, divide litres by 4.54609. So 41.6 litres is 41.6 ÷ 4.54609 = 9.15 gallons.
Watch out for American figures online. A US gallon is smaller, at about 3.785 litres, so the same car shows a lower mpg in US gallons. To turn US mpg into UK mpg, multiply by about 1.201. A car doing 40 mpg in US gallons does about 48 mpg in UK gallons.
The mpg formula
mpg = miles driven ÷ (litres used ÷ 4.54609)
Here is a worked example. You fill up, reset the trip meter, and drive 412 miles. At the next fill the pump takes 41.6 litres.
- Gallons used: 41.6 ÷ 4.54609 = 9.15 gallons
- Economy: 412 ÷ 9.15 = 45.0 mpg
You can also work it in one step: mpg = miles × 4.54609 ÷ litres. That gives 412 × 4.54609 ÷ 41.6 = 45.0 mpg.
Litres per 100 kilometres
Most of Europe measures consumption the other way round, as the litres needed to travel 100 km. Lower is better.
litres per 100 km = litres used ÷ kilometres driven × 100
There are 1.609344 km in a mile, so 412 miles is 663.0 km. Using the same fill: 41.6 ÷ 663.0 × 100 = 6.27 litres per 100 km.
To convert directly between the two, divide 282.48 by the figure you have. So 282.48 ÷ 45 mpg = 6.28 litres per 100 km, and 282.48 ÷ 6.27 litres per 100 km = 45.1 mpg. The tiny differences are just rounding.
| UK mpg | Litres per 100 km |
|---|---|
| 30 | 9.42 |
| 35 | 8.07 |
| 40 | 7.06 |
| 45 | 6.28 |
| 50 | 5.65 |
| 55 | 5.14 |
| 60 | 4.71 |
| 70 | 4.04 |
Cost per mile
Cost per mile tells you what each mile actually costs in fuel, which is more useful than mpg when you are comparing cars, journeys or fuels.
cost per mile = price per litre × litres used ÷ miles driven
Back to the example. At 174.7p a litre, 41.6 litres cost £72.68. Divided by 412 miles, that is 17.6p a mile.
If you already know your mpg, you can skip the fill-up data:
cost per mile = price per litre × 4.54609 ÷ mpg
At 45 mpg and 174.7p: 174.7 × 4.54609 ÷ 45 = 17.6p a mile. Per 100 miles, that is about £17.65.
Worked examples at today's prices
The table uses the UK averages from PetrolNearby on 11 October 2026: 174.7p a litre for E10 petrol and 199.8p for diesel.
| Economy | E10 cost per mile | E10 per 10,000 miles | Diesel cost per mile | Diesel per 10,000 miles |
|---|---|---|---|---|
| 30 mpg | 26.5p | £2,647 | 30.3p | £3,028 |
| 40 mpg | 19.9p | £1,986 | 22.7p | £2,271 |
| 45 mpg | 17.6p | £1,765 | 20.2p | £2,018 |
| 50 mpg | 15.9p | £1,588 | 18.2p | £1,817 |
| 55 mpg | 14.4p | £1,444 | 16.5p | £1,651 |
| 60 mpg | 13.2p | £1,324 | 15.1p | £1,514 |
This shows why comparing on cost per mile matters. Diesel was 25.1p a litre dearer than E10, but a diesel car doing 55 mpg costs 16.5p a mile, which is cheaper than a petrol car doing 45 mpg at 17.6p.
It also shows the value of a few mpg. Going from 40 to 45 mpg on E10 saves about £221 over 10,000 miles. So does the price you pay: at 45 mpg, every 1p a litre adds about 0.1p a mile, or roughly £10 over 10,000 miles. You can find cheaper stations near you on the PetrolNearby home page or check prices by town.
Handling partial fill-ups
You will not always fill the tank. That is fine, as long as you record the partial fills too. The rule: add up all the fuel bought after one full fill, up to and including the next full fill, and divide that into the miles between the two full fills.
| Fill-up | Odometer | Litres | Price | Full tank? |
|---|---|---|---|---|
| 1 | 20,000 | (starting point) | Yes | |
| 2 | 20,180 | 20.0 | 172.9p | No |
| 3 | 20,450 | 22.5 | 174.7p | Yes |
Between the two full fills you drove 450 miles and bought 42.5 litres (20.0 + 22.5). That is 9.35 gallons, or 48.1 mpg. The fuel cost £34.58 plus £39.31, a total of £73.89, which works out at 16.4p a mile.
If you ignored the partial fill and used only the 22.5 litres from the last fill, you would get an impossible 90.9 mpg. That is why partial fills must be counted, and why forgetting to log one throws the figure out.
Why your trip computer may disagree
Most modern cars show an average mpg on the dashboard, and it often does not match your own calculation. That does not mean either is broken.
- It estimates rather than measures. A trip computer works out consumption from how the engine is being run, rather than measuring fuel at the pump. Many drivers find it reads a little optimistic.
- It depends on when you reset it. An average since the last reset may include a long motorway run or a week of short trips that your full-tank calculation does not.
- The odometer is not perfect. Small odometer errors, and tyres that are worn or a different size from standard, change the recorded miles.
- Fills vary. A single full-tank figure can be a litre or so out, which is why averaging several tanks is worth it.
If you want a figure you can trust, the full-tank method over several tanks is the one to use.
Why your economy changes
Your mpg is not a fixed number. The same car can vary noticeably from one tank to the next.
- Season. Engines take longer to warm up in cold weather, heaters and demisters add load, and tyre pressures drop as temperatures fall. Economy is usually worse in winter.
- Short trips. An engine is least efficient when cold, so a run of short journeys can cut economy sharply.
- Speed and driving style. Air resistance rises steeply with speed, so motorway driving at higher speeds uses more fuel. Hard acceleration and late braking waste fuel too.
- Tyres and weight. Under-inflated tyres, heavy loads and roof boxes or bike racks all increase consumption.
- Air conditioning and idling. Both use fuel without moving you further.
- Fuel grade. E10 petrol contains up to 10% ethanol, against up to 5% in E5. Ethanol carries slightly less energy than petrol, so some drivers see a small drop in mpg on E10. E10 vs E5 petrol covers whether paying more for E5 makes sense.
For practical ways to improve your figures, see how to save money on fuel.
Official figures and real-world economy
New cars sold since September 2018 have been tested under the WLTP procedure, which replaced the older NEDC test. WLTP figures are closer to real-world driving than the old ones, but they come from a standard laboratory test, so your own mpg will often be different. Your result depends on your journeys, your driving and the weather.
That is the main reason to calculate your own figure. It tells you what your car does on your roads, which is what actually determines your fuel bill.
Let the PetrolNearby logbook do the sums
If you would rather not keep a notebook and a calculator in the glovebox, the free PetrolNearby logbook does all of the above for you. Each time you fill up, record the litres, the cost and the odometer reading, and say whether you filled the tank.
- Economy appears after two full-tank fill-ups. It needs two full fills to measure the fuel used between them.
- Partial fills count towards the next full one. Record them as partial and the logbook adds their litres in, exactly as in the example above.
- Missed a fill-up? Tick "I missed logging a fill-up before this" and the logbook restarts the calculation instead of showing a wrong figure.
- Your units, your way. In settings, economy can be shown in UK mpg, US mpg, litres per 100 km or km per litre.
- Cost per mile included. It works out what your driving costs from what you actually paid.
Over a few months, the logbook builds a picture of your real economy and spending through the seasons. It pairs well with the price charts and UK fuel prices page, so you can see how changes in pump prices feed into your own costs.
Frequently asked questions
How do I calculate mpg from litres?
Divide the litres you used by 4.54609 to get UK gallons, then divide the miles you drove by that number. For example, 412 miles on 41.6 litres is 412 ÷ (41.6 ÷ 4.54609) = 45.0 mpg.
How do I work out the fuel cost per mile?
Multiply the price per litre by the litres used and divide by the miles driven. If you know your mpg, use price per litre × 4.54609 ÷ mpg instead. At 174.7p a litre and 45 mpg, that is about 17.6p a mile.
Why does my car's trip computer show a higher mpg than my calculation?
Trip computers estimate consumption rather than measuring the fuel you buy, and many read a little optimistic. Differences in when the average was reset and small odometer errors also play a part. The full-tank method averaged over several tanks is the more reliable figure.
Do I have to fill the tank every time?
No. You need full fills at the start and end of each measurement, but partial fills in between are fine as long as you record them and add their litres in. The PetrolNearby logbook handles this automatically.
What is a good mpg?
It depends on the car. A small petrol hatchback, a large SUV and a diesel estate will all return very different figures. The most useful comparison is your own car over time: if your economy drops noticeably with no change in your driving or the season, check tyre pressures and consider a service.
