How to calculate mpg (the full-tank method)
- Fill your tank completely and note the odometer reading.
- Drive normally until your next fill-up.
- Fill the tank completely again, and note the new odometer reading and the gallons it took.
- Divide miles driven by gallons used: 350 miles ÷ 14.0 gallons = 25.0 mpg.
Filling the tank completely both times is what makes this accurate -- it's the only way to know exactly how much fuel was actually used for exactly that distance.
Fuel-weighted average vs. simple average
With several fill-ups logged, the correct overall average is total miles ÷ total gallons, not the average of each fill-up's own mpg:
| Odometer | Fuel added | Miles this interval | mpg this interval |
|---|---|---|---|
| 10,000 | 12 gal | – | – |
| 10,300 | 10 gal | 300 | 30.0 |
| 10,620 | 16 gal | 320 | 20.0 |
Total: 620 miles ÷ 26.0 gallons = 23.8 mpg (fuel-weighted, correct). A simple average of the two per-fill numbers above would instead read 25.0 mpg -- different, because the fill-ups cover different distances and a simple average would let the shorter one skew the result.
mpg to L/100 km, km/L and UK mpg
| mpg (US) | L/100 km | km/L | mpg (UK) |
|---|---|---|---|
| 15 | 15.7 | 6.4 | 18.0 |
| 20 | 11.8 | 8.5 | 24.0 |
| 25 | 9.4 | 10.6 | 30.0 |
| 30 | 7.8 | 12.8 | 36.0 |
| 35 | 6.7 | 14.9 | 42.0 |
| 40 | 5.9 | 17.0 | 48.0 |
| 45 | 5.2 | 19.1 | 54.0 |
| 50 | 4.7 | 21.3 | 60.0 |
| 60 | 3.9 | 25.5 | 72.1 |
| 70 | 3.4 | 29.8 | 84.1 |
Why your mpg differs from the EPA sticker
The window sticker comes from a standardized dynamometer test cycle, not your actual driving. Real-world mpg is usually a bit lower, and varies with driving style, speed, load, tire pressure, and weather -- fueleconomy.gov (the EPA/DOE's own fuel economy site) explains the typical gap and how to close it.
Does tire size affect your odometer, and so your mpg?
A tire size change alters how many times your tires actually turn per mile, which changes what your odometer reports for the same real distance. If you've changed tire sizes, check the speedometer calculator for your true speed and distance error before trusting mpg figures calculated from odometer miles.
Need the cost of a whole trip?
This page is about fuel economy (mpg). For the dollar cost of a specific trip or commute -- including splitting it between passengers -- use the gas trip calculator.
Frequently asked questions
How do I calculate mpg from a fill-up?
Use the "full tank method": fill your tank completely and note the odometer reading. Next time you fill up completely again, note the new odometer reading and how many gallons it took to fill it -- that fuel filled the tank back up after driving the distance in between. Miles driven ÷ gallons used = mpg. Example: 350 miles on 14.0 gallons = 25.0 mpg.
Why should I use the full-tank method instead of trusting the dashboard mpg readout?
Your dashboard's instant/average mpg is an estimate from the engine computer, not a direct fuel measurement, and it resets or drifts depending on the trip computer settings. The full-tank method measures fuel with the pump's own metering (which is legally calibrated) and distance with your odometer, so it's the most reliable number you can get without a dedicated fuel-flow sensor.
What is a fill-up log, and why not just average my mpg over several fill-ups?
A fill-up log records the odometer and gallons at each fill-up so the tool can compute mpg for every interval, then an overall average. That overall average should be total miles ÷ total gallons (the "fuel-weighted" average) -- NOT the simple average of the per-fill mpg numbers. In the example log above, the fuel-weighted average is 23.8 mpg, while a simple average of the two per-fill mpg values reads 25.0 mpg -- a real difference caused by the fills covering different distances.
Why is the fuel-weighted average more accurate than a simple average of per-fill mpg?
mpg is miles per gallon, not gallons per mile, so it does not average linearly across fill-ups of different sizes. A short fill-up with unusually good or bad mpg gets equal weight to a long fill-up in a simple average, distorting the result. Dividing total miles by total gallons weights every mile equally instead of every fill-up equally, which is what you actually want to know: overall, how far did each gallon take you.
How do I convert mpg to L/100 km?
L/100 km = 235.214583 ÷ mpg (US). For 25.0 mpg, that's 9.4 L/100 km. See the full conversion table above for common mpg values.
Why does my mpg not match the EPA sticker on the window?
The EPA window sticker is measured on a dynamometer using a standardized test cycle (city/highway blend), not your actual roads, driving style, load, tire pressure or weather. Real-world mpg is commonly lower than the sticker, especially with more aggressive acceleration, cold weather, highway speeds above 65 mph (aerodynamic drag rises sharply), a heavier load, or underinflated tires -- fueleconomy.gov (the EPA/DOE's own site) publishes typical real-world vs. sticker gaps and driving tips to close them.
Does tire size affect my mpg calculation?
Yes, indirectly, through your odometer. Larger tires than the vehicle was designed with turn fewer times per mile, so a stock odometer under-reads your true distance (and vice versa for smaller tires) -- which throws off both your speedometer and any mpg you calculate from odometer miles. Use the speedometer calculator to check your true speed/distance error after a tire size change before trusting the mpg numbers here.
What's the difference between US mpg and UK (imperial) mpg?
They're both "miles per gallon", but the imperial gallon (4.54609 L) is larger than the US gallon (3.785411784 L), so the same real fuel economy gives a higher UK mpg number. 25.0 US mpg is about 30.0 UK mpg -- the tool above converts automatically.
Can I calculate mpg using liters and kilometers?
Yes -- switch the distance and fuel units above to km and liters and the calculator converts everything for you (mpg US, mpg UK, L/100 km and km/L are all shown regardless of which units you type in).
Where can I find the fuel cost of a whole trip, not just my mpg?
Use the gas trip calculator, which takes your mpg (or L/100 km) plus a distance and gas price and works out the total fuel cost, cost per mile, and cost split between passengers.