MPG Calculator

MPG (miles per gallon) is the standard UK measure of fuel economy: how far a vehicle travels on one gallon of fuel. Crucially, a UK gallon is 4.546 litres — not the US gallon’s 3.785 litres. So American MPG figures (used in car magazines and US-pegged forums) are 20% lower than the equivalent UK MPG figure. A car quoted as 50 MPG in the US is roughly 60 MPG in UK terms. This calculator uses UK gallons throughout. The two ways to calculate MPG are: from a brim-to-brim refill (track miles since last fill, divide by litres pumped, multiply by 4.546), or from your trip computer (which usually displays MPG directly but tends to optimistically overstate real-world by 5-8%). The brim-to-brim method is most accurate. Fill up completely, zero the trip meter, drive normally, fill up again completely at the same pump if possible. Miles ÷ litres × 4.546 = MPG. Repeat over 3-4 tanks for a stable average — single-tank variation can be ±5 MPG depending on traffic, temperature and driving style. To convert between MPG and L/100km (metric standard used across Europe), use 282.5 ÷ MPG = L/100km, and 282.5 ÷ L/100km = MPG. So 40 MPG = 7.06 L/100km; 60 MPG = 4.71 L/100km. Real-world UK MPG averages in 2025: small petrol hatch 45-55, medium petrol saloon 35-45, family diesel SUV 45-55, full hybrid (Toyota Prius, RAV4) 55-70, plug-in hybrid on electric mostly 80-150+ mpg-e (the mpg-e figure depends entirely on charging discipline), full EV 3.5-4.5 miles/kWh which converts to ~150-200 mpg-e equivalent. WLTP brochure figures are typically 15-25% higher than real-world for petrol/diesel and dramatically higher for PHEV (WLTP testing assumes a full battery and limited mileage which is not how most owners drive). What affects MPG. Driving style: gentle acceleration and anticipation can add 15-20% to real-world MPG. Speed: 70 mph uses about 25-30% more fuel than 50 mph. Cold weather: 5-15% worse in winter due to engine warm-up and heater load. Tyre pressure: 5 psi low costs about 2%. Roof bars and bike racks: 5-15% on motorway runs. Short journeys: 25-50% worse than longer trips because the engine never reaches optimal operating temperature. The calculator does not predict your MPG from car specs — use it to measure actual MPG from fuel and mileage data, then plug the result into the Fuel Cost Calculator for journey budgeting. For EV efficiency use the Electric Car Savings Calculator.