The MPG Calculator converts one fill-up's worth of driving, distance travelled and fuel used, into the four fuel-economy figures different countries and dashboards quote: litres per 100 kilometres, US MPG, UK MPG, and kilometres per litre, plus a plain-language efficiency rating. Enter distance in kilometres and fuel in litres.
How It Works
Consumption is computed first in the metric form, then converted with the exact constants for each gallon definition:
The rating bands are the calculator's own: under 6 L/100 km is Excellent, under 8 Good, under 11 Average, under 14 Below Average, and beyond that Poor.
Parameters
| Input | Required | Meaning | |---|---|---| | Distance | Yes | Kilometres travelled on the fuel entered | | Fuel used | Yes | Litres consumed over that distance |
Worked example
A car travels 600 km on 48 litres: consumption is 48 / 600 x 100 = 8.00 L/100 km, which converts to 29.40 US MPG, 35.31 UK MPG, and 12.50 km/L, and rates Average in the calculator's bands (8.00 sits just past the Good boundary at 8). The example's near-boundary rating is a feature to understand, not a rounding trick: these bands are broad comparison labels, and a car at 7.9 and a car at 8.1 are the same car for every practical purpose, whatever the label says.
Why US and UK MPG differ, and why L/100 km is the honest unit
The two MPGs disagree because the gallons do: a US gallon is 3.785 litres, an Imperial gallon 4.546 litres, so the same car travels about 20% more miles per UK gallon and UK figures always look better, a trap in imported car reviews and forum comparisons. The constants in the formulas (235.21 and 282.48) are exactly those gallon definitions folded into the metric conversion. L/100 km avoids a subtler trap inside MPG itself: MPG is a ratio whose improvements compress at the efficient end, so upgrading from 10 to 20 MPG saves far more fuel than upgrading from 30 to 40 over the same distance (the "MPG illusion"), while L/100 km differences are linear in actual fuel burned. For comparing your own car across trips either unit works; for comparing purchase options, consumption per distance is the figure that tracks money.
Measuring your real figure: the fill-to-fill method
The calculator is exact; the measurement discipline is where accuracy is won. Fill the tank completely and reset the trip meter; drive normally; at the next fill, fill completely again and record the litres and the trip distance. That pair, fuel actually consumed over distance actually driven, is the only honest input, and averaging three or four tanks smooths pump shut-off variation, weather, and route mix. Dashboard figures are estimates from injector timing and routinely flatter reality by 5 to 10%; partial fills, top-ups, and "I usually get about" memories produce numbers that are not measurements at all. Note also what a single tank cannot tell you: winter fuel blends, roof boxes, tyre pressure, a dragging brake, and short-trip cold starts all move consumption by amounts larger than most model-year improvements, so a sudden worsening against your own baseline is a maintenance signal worth heeding before it is a statistic.
Frequently Asked Questions
Can I enter miles and gallons directly?
The calculator's maths is built on kilometres and litres (that is what its conversion constants assume), so convert first: miles x 1.609 = kilometres, US gallons x 3.785 = litres. Entering miles and gallons straight in produces the US MPG of a different, fictional car.
Why is my winter figure so much worse?
Cold starts enrich the mixture, winter fuel blends carry slightly less energy, air is denser, tyres lose pressure, and lubricants thicken; 10 to 20% worse winter consumption is normal physics, not necessarily a fault. Judge changes against the same season last year.
Is the rating an official efficiency classification?
No. It is this calculator's broad banding of the consumption you measured, useful for orientation. Official ratings (WLTP, EPA) come from standardized test cycles, which is why they are comparable across cars but rarely match any individual's fill-to-fill reality.