MPG Calculator
Calculate real-world fuel economy from your fill-ups and convert between MPG, L/100km and km/L.
Your details
Fill to the same click both times — that is what makes tank-to-tank measurement accurate.
Cost
Per gallon or per litre, matching your units above.
This calculator runs entirely on your device. Nothing you enter is uploaded, stored, or sold. How this works
Fuel economy
28.1 MPG
8.4 L/100km · 11.9 km/L · 33.7 MPG imperial
Includes your inputs, the full breakdown, and every row of the table.
US MPG
28.07
Imperial MPG
33.71
L per 100 km
8.38
Cost per mile
$0.12
- Distance
- 320.0 miles
- Fuel used
- 11.40 gallons
- US MPG
- 28.07
- Imperial MPG
- 33.71
- L/100 km
- 8.38
- km/L
- 11.93
- Fuel price
- $3.35
- Cost per mile
- $0.12
- Annual fuel cost at 12,000 miles
- $1,432
| Economy | Cost per mile | Annual cost | Saved vs yours |
|---|---|---|---|
| 20 MPG | $0.17 | $2,010 | -$578 |
| 25 MPG | $0.13 | $1,608 | -$176 |
| 30 MPG | $0.11 | $1,340 | $92 |
| 35 MPG | $0.10 | $1,149 | $284 |
| 40 MPG | $0.08 | $1,005 | $427 |
| 45 MPG | $0.07 | $893 | $539 |
| 50 MPG | $0.07 | $804 | $628 |
What this means
- Imperial gallons are about twenty percent larger than US gallons, which is why UK figures always look better for the same car. Comparing across the two without converting is the single most common fuel economy mistake.
- L/100km is inverted relative to MPG — lower is better — and it is the more honest measure for comparing savings. Going from 15 to 20 MPG saves far more fuel than going from 40 to 45, even though both are a five MPG improvement.
- Measuring tank to tank beats trusting the dashboard readout, which is typically optimistic by a few percent. Fill to the same click each time and take an average over several tanks to smooth out variation.
How the mpg calculation works
Fuel economy is measured three different ways around the world and they are not interchangeable. US miles per gallon uses a gallon of 3.785 litres; imperial MPG uses 4.546 litres, making the same car appear roughly twenty percent more efficient. Most of the rest of the world uses litres per hundred kilometres, which inverts the relationship entirely so that lower numbers are better.
That inversion is not just a formatting difference — it changes how you should think about savings. MPG compresses the important end of the scale. Improving from fifteen to twenty MPG saves about one and a half gallons per hundred miles, while improving from forty to forty-five saves under three tenths of a gallon. The five-unit change looks identical in MPG and is five times more valuable in the first case. L/100km makes this obvious immediately, which is a genuine argument for the metric convention.
Measuring your own economy tank to tank is more reliable than trusting the dashboard computer, which is generally optimistic by a few percent and on some manufacturers considerably more. The method is simple: fill the tank to the pump's first automatic click, reset the trip meter, drive normally, then refill to the same click and divide distance by fuel added. Averaging several tanks smooths out variation from traffic, weather and terrain.
Frequently asked questions
Why is my MPG lower than the manufacturer's figure?
Official figures come from standardised laboratory cycles that involve gentle acceleration and no extremes of weather. Real driving with cold starts, short trips, air conditioning, roof racks and traffic typically lands five to fifteen percent below the sticker, and considerably lower in winter.
What is the difference between US and imperial MPG?
The gallon. A US gallon is 3.785 litres and an imperial gallon is 4.546, so the same vehicle reads about twenty percent higher in imperial MPG. Any comparison between US and UK figures needs converting first or it is meaningless.
Is L/100km better than MPG?
As a measure of fuel consumed per distance it is more useful, because it scales linearly with actual fuel use. MPG compresses the low-efficiency end of the scale, which makes small improvements to inefficient vehicles look less valuable than they really are.
How can I improve my fuel economy?
Steady speeds and gentle acceleration matter most, followed by correct tyre pressure, removing roof racks and unnecessary weight, and combining short trips so the engine reaches operating temperature. Driving at seventy rather than eighty typically gains around ten percent on its own.
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Disclaimer. This calculator is provided for general information and educational purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Results are estimates based on the inputs you provide and the assumptions described above. Confirm any figure with a qualified professional before acting on it.