MPG calculator: how to work out your miles per gallon

Miles per gallon is the number everyone quotes and few people actually measure. Knowing your true MPG tells you what your fuel really costs, flags a developing mechanical problem before it becomes expensive, and lets you compare cars honestly. The good news: measuring it needs nothing more than a full tank, your odometer and a receipt.

How to calculate MPG

The method, step by step:

• Fill the tank completely and note the odometer reading (or reset a trip meter to zero).
• Drive normally until you next refuel — ideally at least half a tank for a reliable figure.
• Fill up again and record how many litres it took and the new mileage.
• Do the maths: miles driven = new mileage − old mileage; gallons used = litres ÷ 4.546; then MPG = miles ÷ gallons.

The trick is that the second fill-up tells you exactly how much fuel the trip consumed, because you’re replacing precisely what you burned. Fill to the same click-off point each time for consistency.

Worked example

You start on a full tank at 24,000 miles, drive for a couple of weeks, and refill at 24,350 miles taking 35 litres.

• Miles driven: 24,350 − 24,000 = 350 miles.
• Gallons used: 35 ÷ 4.546 = 7.70 gallons.
• MPG: 350 ÷ 7.70 = 45.5 MPG.

Once you know your MPG, you can turn it straight into money with our fuel cost calculator, or express your whole running cost with the cost per mile calculator.

MPG vs litres per 100 km

The rest of the world measures economy the other way round — litres per 100 km — and the two aren’t just different units, they run in opposite directions. With MPG, higher is better. With L/100 km, lower is better, because it counts fuel burned rather than distance covered. To convert:

L/100 km = 282.5 ÷ UK MPG, and UK MPG = 282.5 ÷ L/100 km

So 50 UK MPG is about 5.65 L/100 km, and 6 L/100 km is about 47 MPG. Beware the gallon difference when comparing figures online: a UK gallon (4.546 litres) is larger than a US gallon (3.785 litres), which makes UK MPG numbers roughly 20% higher than US ones for identical real-world economy. If you use the US constant instead, it’s L/100 km = 235.2 ÷ US MPG.

Real vs official economy

The MPG on the window sticker comes from the WLTP lab test — a standardised, repeatable cycle that’s useful for comparing cars but rarely matches your commute. Real driving adds cold starts, stop-start town traffic, high motorway speeds, heavy loads, roof bars, cold weather and air-conditioning, all of which push consumption up. As a result, real-world MPG typically comes in 10–25% below the official figure, and short urban journeys can be far worse because the engine never fully warms up. That’s exactly why measuring your own MPG matters: it’s the only number that reflects how you actually drive.

How to improve your MPG

• Ease off. Gentle acceleration and braking, and reading the road ahead, make the biggest difference.
• Slow down a little on the motorway. Drag rises sharply with speed, so 65–70 uses noticeably less than 80.
• Check tyre pressures. Under-inflated tyres add rolling resistance and cost you fuel.
• Drop the dead weight. Remove roof bars and clear the boot — extra weight and drag both bite.
• Keep up with servicing. Clean filters, fresh oil and healthy tyres all help economy.
• Track it. Watching MPG tank by tank in CarBudget tells you what actually works — and warns you when something’s wrong.

Sources and methodology

The conversions and ranges here rely on standard published figures:

• Gallon and litre definitions (UK gallon 4.546 L, US gallon 3.785 L) and the MPG ↔ L/100 km constants: official UK unit definitions.
• WLTP test basis for official economy figures: UK Vehicle Certification Agency (VCA) fuel-economy data.
• Real-world vs official economy gap and fuel-saving driving tips: RAC and AA economy guidance.