The short answer
Fill the tank to full and note the odometer. At the next full fill-up, divide the miles driven by the gallons you put in: 312 miles ÷ 10.4 gallons = 30 mpg. Fuel Studio’s “Fill-ups” tab does this for every tank, keeps a running average and shows your cost per 100 miles.
Open the free toolThe full-tank method, step by step
Fill up to full
Fill until the pump clicks off the first time. Do not top up – the same click point each time makes the method accurate.
Note the odometer
Write down the odometer reading, or enter it in Fuel Studio’s “Fill-ups” tab with the date and tick “Filled up to full”.
Drive normally
Use the car as you usually do. Partial fill-ups in between are fine – enter them too, without the “Filled up to full” tick.
Fill up to full again
Enter the gallons and the odometer. Fuel Studio adds every gallon since the last full tank and divides the distance by it.
A worked example
Say you filled up to full at 21,400 miles. A week later you added 4 gallons on the way, and at 21,712 miles you filled up to full again with 6.4 gallons. The distance is 312 miles, the fuel 4 + 6.4 = 10.4 gallons, so your real mileage is 30 mpg.
If you paid $3.50 a gallon, those 312 miles cost $36.40 – about $11.67 per 100 miles. Fuel Studio shows the same three numbers on top of the diary: average consumption, last fill-up and cost per 100 miles.
mpg, UK mpg and l/100 km
US mpg uses the US gallon (3.785 litres), UK mpg the larger imperial gallon (4.546 litres), so the same car shows a higher number in UK mpg. Most of the world uses litres per 100 km, where lower is better.
| US mpg | UK mpg | l/100 km |
|---|---|---|
| 20 | 24.0 | 11.8 |
| 30 | 36.0 | 7.8 |
| 40 | 48.0 | 5.9 |
| 50 | 60.0 | 4.7 |
Why your mileage is worse than the sticker
- Short trips: a cold engine uses the most fuel, and many errands never warm it up.
- Speed: fuel use climbs steeply above highway cruising speeds.
- Cold weather and heating, the A/C in summer, and winter fuel blends.
- Load and drag: passengers, luggage, roof boxes and bike racks.
- Tyre pressure below the value on the door sticker.
Mistakes that make the result wrong
- Topping up after the first click: the tank is fuller than last time, so this fill-up looks thirstier and the next one too frugal.
- Forgetting a partial fill-up: the gallons are missing from the total and your mpg looks better than it is. Enter every fill-up, ticked or not.
- Reading the trip odometer after a reset: use the main odometer, or make sure the trip counter really started at the last full tank.
- Judging by a single tank: one highway trip or one cold week skews the number. The average over several full tanks is the honest figure.
What your mileage costs per year
Mileage turns into money quickly. At 12,000 miles a year and $3.50 a gallon, a car doing 25 mpg uses 480 gallons and costs $1,680 in gas; at 32 mpg it is 375 gallons and $1,312.50. The difference of seven mpg is worth about $370 a year.
Fuel Studio’s diary shows your own number: “Cost per 100 miles” on top and your spending per month in the chart. The “EV vs gas” tab uses the same kind of figures to compare an electric car with yours.
How to improve your mpg – and check that it worked
Pick one change at a time: check tyre pressure monthly, remove the roof box, combine errands into one trip or drive the highway a little slower. Keep logging fill-ups for a few weeks before and after. Because Fuel Studio keeps every tank, you can compare the average before and after the change instead of trusting a feeling.
The diary also shows your spending per month as a chart. Export it as CSV at any time for a spreadsheet or to move it to another device – it is stored only in your browser.
Questions you might still have
How do I calculate mpg?
Divide the miles driven between two full tanks by the gallons you put in to fill the tank again. Fuel Studio does it for every fill-up and keeps a running average.
Why should I fill up to full each time?
The method assumes the tank is equally full at the start and the end. Filling until the first click both times makes that true; topping up adds errors.
Is the trip computer’s mpg accurate?
It is a useful guide, but many trip computers read a little optimistic. The full-tank method uses the fuel you actually paid for.
Can I log fill-ups on my phone?
Yes. Fuel Studio works in any current phone browser; the diary stays on that device. Use CSV export and import to move it.
What is good gas mileage?
It depends on the car: a small hybrid and a full-size pickup are built for very different numbers. Compare your result with the official rating for your model on fueleconomy.gov, and with your own average over time.
Does a diesel or a hybrid change the method?
No. The full-tank method works for any fuel car. Only plug-in hybrids that also charge from the grid need the electricity counted separately.
Sources & useful details
- U.S. Department of Energy and EPA — fuel economy information
- ADAC — fuel consumption and saving tips
- Ministère de la Transition écologique — economical driving tips
- IDAE — efficient driving
These guides are written from the current LoschaZ tools and the sources listed above. Examples are illustrations, not tests on every device or promises of an identical result.
