Gasoline Vehicles
Gasoline & Petrol MPG Calculator
This calculator works out the real-world fuel economy of a gasoline vehicle from your odometer readings and what you actually put in the tank. Manufacturer figures come from a controlled test cycle; this gives you the number your car returns in the driving you actually do.
Enter the distance covered and the fuel volume in US gallons, UK (Imperial) gallons, or litres. You get your MPG, the metric equivalents (L/100km and km/L), miles per litre, and — if you add a pump price — your cost per mile. Measure over a full tank rather than a single short trip; one cold start or one motorway run will skew a short sample badly.
Gasoline or petrol — the word changes, and so does the gallon
Gasoline and petrol are the same fuel. The word you use depends on where you are, and on its own it changes nothing about the arithmetic. The gallon is the part that catches people out.
The US gallon holds 3.78541 litres. The UK (Imperial) gallon holds 4.54609 — about 20% more. So the same car, burning the same fuel over the same road, is rated 40 MPG in Britain and 33 MPG in America. Neither number is wrong; they are measured against different-sized gallons.
This matters whenever you compare a figure you read against a figure you calculated. A US driver looking at a British review, or a UK driver reading an EPA rating, will be roughly 20% out unless they convert. Pick the right unit in the calculator below and the label above your result tells you which gallon you are working in. To go the other way, the UK MPG calculator and the MPG to L/100km converter handle the conversion directly.
How to Use
- Enter the miles driven between the two fill-ups.
- Choose your fuel unit: US gallons, UK gallons, or litres.
- Enter the gasoline used — the amount of the second fill.
- Optionally add the fuel price per unit to get cost per mile.
- Click Calculate Gasoline MPG (or press Enter).
Related tools: MPG Calculator, Diesel MPG Calculator, UK MPG Calculator, Miles Per Litre Calculator.
Got Questions?
Gasoline & Petrol MPG Calculator FAQ
The fuel is the same — gasoline is the American word, petrol the British one. The number is not the same, because the gallon differs. A car rated 40 MPG in the UK is 33 MPG in the US, since a UK gallon holds 4.54609 litres against the US gallon’s 3.78541. Always check which gallon a figure refers to before comparing two cars.
Fill the tank completely and note the odometer. Drive normally until you need fuel again, then fill completely a second time. Divide the miles covered between fill-ups by the gallons it took to refill. That is your real miles per gallon for those conditions.
The model year 2024 average across all new US light-duty vehicles was 27.2 MPG. Anything from 30 MPG up is therefore above average, and 40+ is excellent for a non-hybrid. UK figures run higher on the same car because the Imperial gallon is larger: 40–50 UK MPG is the equivalent band.
Manufacturer MPG figures come from lab tests. Real-world driving — cold starts, air conditioning, traffic, speed variation, and tyre pressure — reduces economy by 10–25%.
Burning one US gallon of gasoline releases 8,887 g of CO₂ — about 19.6 lb. Per litre that is 2.35 kg, slightly less than diesel’s 2.68 kg. A car averaging 30 US MPG emits around 296 g of CO₂ per mile.
Only if your engine is tuned for it. Premium gasoline (91–93 AKI in the US, 97–99 RON in the UK) can improve economy by 2–4% in an engine that requires it, and does essentially nothing in one that does not. Regular 87 AKI or 95 RON is correct for most cars.