How do I calculate MPG accurately?

Fill your tank to the brim, reset your trip meter, drive normally until you need fuel again, then fill to the brim again. Divide the miles driven by the litres used, then multiply by 4.54609 to convert to UK MPG. This brim-to-brim method is the most accurate way to measure real-world fuel consumption.

What is a good MPG for a car?

For a modern petrol car, 40-50 mpg is good for mixed driving. Diesel cars typically achieve 50-65 mpg. City driving will be lower and motorway driving higher. Older cars and larger engines will naturally return lower figures.

Why is my MPG different from the manufacturer figure?

Manufacturer figures are measured under controlled WLTP test conditions. Real-world driving involves cold starts, traffic, weather, and varying road conditions that typically reduce economy by 10-20% compared to the official figure.

What is the difference between UK and US MPG?

A UK (imperial) gallon is 4.546 litres while a US gallon is only 3.785 litres. This means UK MPG figures are about 20% higher than US MPG for the same fuel consumption. Always check which gallon is being used when comparing figures.

How do I convert MPG to litres per 100km?

Divide 282.481 by your UK MPG figure. For example, 45 mpg = 282.481 / 45 = 6.3 L/100km. This conversion uses the UK imperial gallon. For US MPG, divide 235.215 by the US MPG value instead.

CarCalcs

MPG Calculator

Calculate your car's real-world miles per gallon from distance driven and fuel used.

Distance Driven

Fuel Used

How We Calculate This

This calculator converts your distance driven and fuel used into multiple fuel economy formats.

The Formulas

UK MPG = Miles driven / (Litres used / 4.54609)
US MPG = Miles driven / (Litres used / 3.78541)
L/100km = (Litres used / (Miles x 1.60934)) x 100
Miles per litre = Miles driven / Litres used

Tips for Better MPG

Maintain correct tyre pressures — underinflated tyres increase rolling resistance
Avoid harsh acceleration and braking
Remove unnecessary weight and roof boxes when not in use
Use cruise control on motorways

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Actual figures may vary based on driving conditions, vehicle specifications, and other factors.