How to Calculate MPG: The Brim-to-Brim Method

To calculate mpg, fill the tank, note the trip meter, drive normally, then fill to the brim again. MPG is the miles driven divided by the litres added, times 4.54609. For example, 612 miles on 52.4 litres is 53.1 mpg. At the diesel price for the week of 26 October 2026, that is 17.1p a mile.

How do you work out mpg step by step?

Use the brim-to-brim method. It needs a trip meter, a pump receipt and two full tanks.

1. Fill the tank until the pump clicks off. Do not top up after the click. Note the odometer reading, or reset the trip meter to zero.
2. Drive as you normally would. Do not change your route or your habits for the test.
3. Fill the tank again at the same pump if you can, and stop at the first click. Write down the litres added from the receipt.
4. Note the miles driven since the first fill.
5. Convert litres to gallons by dividing by 4.54609, then divide the miles by the gallons.

As a formula: mpg = miles / (litres / 4.54609), which is the same as miles / litres x 4.54609.

Worked example: you drive 612 miles and the second fill is 52.4 litres. That is 52.4 / 4.54609 = 11.53 gallons. Then 612 / 11.53 = 53.1 mpg.

Why use the same pump and the same stop point?

The brim-to-brim method depends on the tank being equally full at both ends. Pumps click off at slightly different points, and the angle of the vehicle on the forecourt changes how much fits in. A different pump, a sloping forecourt or a top-up after the click can move the answer by a litre or more, which is a big error on a single tank.

The fix is volume. Run the test over several tanks, or at least 300 to 400 miles, so that one litre of error is a small share of the total. Then add up all the miles and all the litres across the period and divide once. Do not average the individual mpg figures, because tanks with fewer miles would carry too much weight.

Why can the dashboard mpg differ from the real figure?

The dashboard figure is an estimate calculated by the vehicle’s computer from the fuel injected and the distance travelled. It does not measure the fuel in the tank, and it does not know what the pump actually delivered. Your brim-to-brim figure measures what you paid for.

Several things can open a gap between the two. The computer’s distance may differ slightly from the true distance, for example if tyre wear or a different tyre size changes the rolling circumference. The injected fuel is estimated, not weighed. Some displays reset at every start or after a set time, so the number you see is only a window, not your average. If the two figures disagree, trust the one based on receipts.

How do you convert mpg to cost per mile?

Pence per mile = price per litre in pence x 4.54609 / mpg. Using the DESNZ weekly road fuel prices for the week of 26 October 2026, diesel was 199.22p a litre and petrol was 174.07p.

For the 53.1 mpg example above, a diesel vehicle costs 199.22 x 4.54609 / 53.1 = 17.1p a mile. A petrol vehicle at the same mpg costs 174.07 x 4.54609 / 53.1 = 14.9p a mile.

The table shows how cost per mile moves with mpg at the same week’s prices:

MPG	Litres per 100 miles	Petrol cost per mile	Diesel cost per mile
30	15.2	26.4p	30.2p
35	13.0	22.6p	25.9p
40	11.4	19.8p	22.6p
45	10.1	17.6p	20.1p
50	9.1	15.8p	18.1p
55	8.3	14.4p	16.5p
60	7.6	13.2p	15.1p

Once you have the pence per mile, multiply by the miles you drive to see the annual fuel bill. For a 100-mile trip, see how to calculate fuel cost for a journey.

Can you calculate mpg without filling up twice?

Yes, using fuel card or telematics data. Take the litres bought across a period from your fuel card statements, take the miles from odometer readings at the start and end, and use the same formula. This is the same sum as brim-to-brim, only over a longer period and without the tank-level risk.

For a fleet, this is the best method. Do it per vehicle, because averaging a van and a car tells you nothing useful about either. Watch for a card used in the wrong vehicle, or for fuel put into cans, as these distort the result.

How do fleets use mpg figures?

Fleets use mpg to compare vehicles, spot faults and set targets. A vehicle that drops several mpg against its own history may have a tyre pressure problem, a mechanical fault, a heavier load or a driver who is going faster. Comparing the same vehicle over time is more reliable than comparing different vehicles.

Official mpg is a lab figure and rarely matches on-road use. See why real-world mpg is lower than official for the reasons. If you need the figure in another unit, such as litres per 100 km, our MPG to litres per 100km guide has the conversion.

The fuel cost calculator takes your measured mpg and shows the cost per mile and the annual bill, using this week’s prices.

Frequently Asked Questions

What is the formula for mpg?

MPG = miles driven / (litres used / 4.54609). The 4.54609 is the number of litres in a UK gallon. A vehicle that travels 612 miles on 52.4 litres does 53.1 mpg.

Is the dashboard mpg accurate?

It is an estimate, not a measurement. The computer calculates it from the fuel it injects and the distance it records, and it can differ from what you actually put in the tank. Measure over several tanks using receipts if you need a figure you can rely on.

How many miles do I need to calculate mpg accurately?

At least 300 to 400 miles, and more is better. A longer run makes the error from a different pump click-off point a small share of the total. Add up the miles and litres across all the tanks and divide once.

How do I turn mpg into cost per mile?

Multiply the price per litre in pence by 4.54609, then divide by your mpg. At 199.22p a litre for diesel and 53.1 mpg that is 17.1p a mile. Prices are from DESNZ for the week of 26 October 2026.

Does a UK gallon equal a US gallon?

No. A UK gallon is 4.54609 litres and a US gallon is 3.785411784 litres. This matters when you read mpg figures from American sources, which look lower than UK figures for the same vehicle. See UK vs US mpg.

Cut your fleet’s fuel bill

Find your own number with the fuel cost calculator. Enter your measured mpg and annual mileage, and it shows the cost per mile and the yearly fuel bill using this week’s DESNZ prices.

A speed cap saves fuel on every journey without the driver having to remember it. The Department for Transport’s fuel consumption curves (TAG data book v2.03, May 2026) show a diesel van using 14.5% less fuel per mile at 60 mph than at 68 mph. How much of that reaches your bill depends on how many of your miles are driven above the cap.

Talk to us. AutoKontrol has made speed limiters in Britain since 1985, with nationwide installation where we come to you. Book a free consultation or get a quote.