How to work out your car’s real mpg

Updated 23 September 2026

To work out your mpg, divide the miles you drove by the gallons you put in. In the UK, where fuel is sold by the litre, divide the litres by 4.546 first. For a reliable figure, fill up brim to brim over several tanks. Here is how, step by step, with an example, and why the result won’t match your trip computer or the official figure.

The formula

UK: mpg = miles ÷ (litres ÷ 4.54609).
US: mpg = miles ÷ gallons.
L/100 km = 282.48 ÷ UK mpg, or 235.21 ÷ US mpg.

An imperial gallon is 4.54609 litres and a US gallon is 3.785411784, so the same car shows a UK mpg figure 20% higher than its US one. If you’d rather not do the maths, the MPG calculator gives you UK and US mpg, L/100 km and your cost per mile.

The brim-to-brim method, step by step

Fill the tank until the pump first clicks off. Don’t top up after that.
Reset the trip meter before you leave the forecourt.
Drive as you normally do until you need to fill up again. The more miles, the less any filling error matters.
Fill up to the first click again and note the fuel on the receipt and the miles on the trip meter.
Divide the miles by the gallons (in the UK, convert the litres first).

The pump doesn’t always click off at the same point: the angle of the car, the fill speed and fuel foam all play a part. That’s why it’s best to use the same station, and the same pump if you can, and to measure several tanks in a row. A quarter of a gallon either way on a 466-mile tank moves the result by about 0.8 mpg; over three tanks, by about 0.3 mpg.

To measure several tanks, add up all the fuel you put in after the first fill-up and all the miles, and divide the totals. If you add fuel without filling up in between, include it too: what matters is starting and finishing with a full tank.

Example over three tanks

Fuel economy measured over three fill-ups
Fill-up	Miles	Litres (UK)	mpg (UK)	Gallons (US)	mpg (US)
1	466	45.0	47.1	11.9	39.2
2	423	42.3	45.5	11.2	37.8
3	510	48.1	48.2	12.7	40.2
Total	1,399	135.4	47.0	35.8	39.1

The total is not the average of the three figures: divide total miles by total fuel.

Each tank gives a slightly different figure, depending on the driving and where the pump clicked off. The total, 47 mpg UK (39 mpg US, or 6.0 L/100 km), is the reliable figure.

Your trip computer

The trip computer shows your average mpg since it was last reset, and often your instant figure. It’s handy but not always accurate. Germany’s ADAC compared the trip computers of 80 models with the fuel use it measured in its tests: six were accurate to the decimal place and another 19 were within 2%. The ADAC considers up to 5% acceptable. Most combustion-engined cars showed higher consumption than they really used: only 13 out of 64 showed less. The least accurate, 13.8% out, displayed 6.6 L/100 km (42.8 mpg UK) when the car used 5.8 (48.7 mpg UK).

To check yours, reset it at the same time as the trip meter when you fill up, and compare its average with your brim-to-brim figure.

The official figure

The official figure (WLTP in the UK and Europe, the EPA label in the US) comes from a standard test and is meant for comparing models, not for predicting what you’ll get. In Europe, real-world fuel use is on average 17–20% higher than WLTP; the US EPA label is already adjusted and much closer. More in real-world vs official fuel economy.

What changes your mpg

Where you drive: town driving and queues use more than open roads. How much, in how to cut your commuting fuel costs.
Speed: across 74 vehicles, ORNL found fuel economy drops 12.4% from 50 to 60 mph, 14% from 60 to 70 mph and 15.4% from 70 to 80 mph.
Luggage, roof bars and roof boxes: roof boxes and luggage.
Air conditioning, especially in town: air conditioning or windows down.
Cold weather: according to fueleconomy.gov, a petrol car’s city mpg is about 15% lower at 20°F (−7°C) than at 77°F (25°C), and up to 24% lower on short 3–4 mile trips.

Your real mpg is an average of how you use the car. Measure in summer and winter, or on holiday and in daily driving, and you’ll get different figures.

What to do with your figure

Work it out in the MPG calculator, then cost a journey in the trip fuel cost calculator: enter your mpg and choose “My real figure”. Because it’s your actual fuel economy, we don’t correct it as we do official figures.

Frequently asked questions

How do I convert UK mpg to US mpg?

Multiply UK mpg by 0.833 to get US mpg, or US mpg by 1.201 to get UK mpg. The difference comes from the size of the gallon: 4.54609 litres in the UK and 3.785411784 in the US.

Is it normal for my mpg to drop in winter?

Yes. Cold oil means more friction, the engine takes longer to warm up and the heater and demisters use energy. According to fueleconomy.gov, a petrol car’s city mpg is about 15% lower at 20°F (−7°C) than at 77°F (25°C).

How many tanks should I measure?

One tank gives you a first figure, but three or more in a row, dividing total miles by total fuel, is better: the filling error matters less each time.

What if I don’t fill the tank completely?

A partial fill-up can’t be measured on its own. Add its fuel to the total and keep going until the next full fill-up.

Why does my trip computer show a different mpg?

It estimates fuel use from what the engine injects and isn’t always right. In the ADAC test, most showed slightly higher consumption than the car really used. Brim to brim is the benchmark.