The figure on the window sticker comes from a laboratory. Your own comes from the pump: the distance since the last full tank, divided by the fuel it took to fill it again. Enter both – in miles or kilometers, in liters, US gallons or UK gallons – and the calculator shows the result in every measure used around the world, the cost per mile or kilometer at your fuel price, and the CO₂ the fuel puts out.
Example (US): 312 miles since the last fill-up, 9.6 gallons to fill up again at $3.40 a gallon. That is 32.5 mpg (7.24 L/100 km), and fuel costs $0.105 a mile. Example (UK): 420 miles on 48 liters is 39.8 mpg in UK gallons, or 7.10 L/100 km.
Fuel economy conversion table
| L/100 km | km per liter | US mpg | UK mpg | CO₂, gasoline | CO₂, diesel |
|---|---|---|---|---|---|
| 3 | 33.3 | 78.4 | 94.2 | 70 g/km | 81 g/km |
| 4 | 25.0 | 58.8 | 70.6 | 94 g/km | 108 g/km |
| 5 | 20.0 | 47.0 | 56.5 | 117 g/km | 134 g/km |
| 6 | 16.7 | 39.2 | 47.1 | 141 g/km | 161 g/km |
| 7 | 14.3 | 33.6 | 40.4 | 164 g/km | 188 g/km |
| 8 | 12.5 | 29.4 | 35.3 | 188 g/km | 215 g/km |
| 9 | 11.1 | 26.1 | 31.4 | 211 g/km | 242 g/km |
| 10 | 10.0 | 23.5 | 28.2 | 235 g/km | 269 g/km |
| 12 | 8.3 | 19.6 | 23.5 | 282 g/km | 323 g/km |
| 15 | 6.7 | 15.7 | 18.8 | 352 g/km | 403 g/km |
| 20 | 5.0 | 11.8 | 14.1 | 470 g/km | 538 g/km |
And from US mpg:
| US mpg | L/100 km | UK mpg |
|---|---|---|
| 20 | 11.76 | 24.0 |
| 25 | 9.41 | 30.0 |
| 30 | 7.84 | 36.0 |
| 35 | 6.72 | 42.0 |
| 40 | 5.88 | 48.0 |
| 50 | 4.70 | 60.0 |
Two ways of looking at the same thing
Most of the world measures consumption – liters used per 100 km. The US and UK measure economy – miles per gallon. They behave differently: going from 15 to 20 mpg saves more fuel than going from 40 to 50 mpg. Over 10,000 miles, the first change saves 167 US gallons; the second, 50. That is why L/100 km, where the savings add up in a straight line, is easier for comparing cars.
The CO₂ from your tank
Burning a US gallon of gasoline releases about 8,887 g of CO₂, and a gallon of diesel about 10,180 g, according to the US EPA – 2.35 and 2.69 kg per liter. The calculator multiplies that by the fuel you use per mile or kilometer. It counts only what comes out of the exhaust, not the oil production and refining behind it.
Getting a reliable number
- Fill up at the same pump if you can, and stop at the first automatic click each time.
- Average two or three tanks, or a whole month of driving.
- Short trips, cold weather, heavy loads, roof boxes and speeds above 60 mph (100 km/h) all raise consumption noticeably.
For a whole journey, the travel time calculator gives the hours on the road. Once zet4u is saved on your phone, this calculator works offline, at a gas station with no signal too.
Frequently asked questions
How do I calculate my mpg?
Fill the tank to the brim and reset the trip meter. Drive normally until the next fill-up, fill to the brim again, and divide the miles on the trip meter by the gallons you just put in. 312 miles on 9.6 gallons is 32.5 mpg. Over two or three tanks the number gets steadier.
How do I convert mpg to L/100 km?
Divide 235.21 by US mpg, or 282.48 by UK mpg. 30 US mpg = 7.84 L/100 km; 40 UK mpg = 7.06 L/100 km. The same number works backwards: 235.21 ÷ L/100 km = US mpg.
Why is UK mpg higher than US mpg for the same car?
Because the UK (imperial) gallon is bigger: 4.546 liters against 3.785. The same car that does 30 mpg in the US does 36 mpg in UK gallons. Comparing a British and an American review, convert one of them first.
Why is my real fuel economy worse than the official figure?
Official figures (the EPA label in the US, WLTP in Europe and the UK) come from standard laboratory test cycles, so that every car is measured the same way. Real driving adds traffic, cold starts, hills, air conditioning, roof racks and speed, and most drivers see a higher consumption than the label.
Sources
- US EPA: Greenhouse gas emissions from a typical passenger vehicle (CO₂ per gallon of gasoline and diesel)
- NIST Special Publication 811: Guide for the Use of the SI, Appendix B (mile, US gallon)
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