What is the difference between UK and US MPG?

UK MPG uses imperial gallons (4.546 litres) while US MPG uses US gallons (3.785 litres). This means UK MPG figures are always higher than US figures for the same vehicle. A car rated at 45 mpg (imperial) would be approximately 37.5 mpg (US). This calculator uses UK imperial gallons.

What is a good MPG for a UK car?

For petrol cars, 40-50 mpg (imperial) is considered good. Modern diesel cars often achieve 50-65 mpg. Hybrid vehicles typically manage 55-70 mpg in mixed driving. The figures vary with driving style, road conditions, and vehicle condition.

How do I convert MPG to L/100km?

To convert UK imperial MPG to L/100km, divide 282.48 by the MPG figure. For example, 45 mpg = 282.48 / 45 = 6.3 L/100km. Lower L/100km values indicate better fuel economy, while higher MPG values indicate better economy.

Why is my real-world MPG lower than the official figure?

Official WLTP fuel economy figures are measured under controlled lab conditions. Real-world MPG is typically 10-20% lower due to traffic, weather, driving style, air conditioning use, and carrying passengers or luggage. Motorway cruising at higher speeds also reduces fuel economy compared to moderate-speed driving.

Fuel Economy Calculator

Calculate MPG (imperial), L/100km, and cost per mile for UK drivers

Published by UltraCalculatorsLast reviewed: 29 Mar 2026v1.0Sources:AA Fuel Prices RAC Fuel Watch Gov.uk Energy and Environment DataReport an issue

Fuel economy is one of the most important running costs for UK drivers, yet many motorists are unsure how to calculate their actual miles per gallon (MPG) or compare it with the manufacturer's quoted figure. With fuel prices fluctuating regularly, understanding exactly how much each journey costs helps with budgeting and choosing between vehicles. This fuel economy calculator uses UK imperial gallons (4.546 litres per gallon), which is the standard measurement for MPG in Britain. This is an important distinction because US gallons are smaller (3.785 litres), meaning American MPG figures are always lower than UK figures for the same vehicle. If you are reading fuel economy data from a US source, the numbers will not match UK expectations unless converted. The calculator takes three simple inputs: distance driven in miles, fuel used in litres, and the current price per litre. From these, it computes your imperial MPG, the metric equivalent in litres per 100 kilometres (L/100km), total fuel cost for the journey, and the cost per mile. These figures help you compare your vehicle's real-world efficiency against official WLTP ratings, track fuel costs over time, and estimate running costs for planned journeys. Whether you are comparing petrol versus diesel, evaluating a potential purchase, or simply keeping track of your monthly fuel spend, this tool gives you the numbers you need in seconds.

How to Use This Calculator

To calculate your fuel economy: 1. Enter the distance driven in miles. This is the total distance covered since you last filled up, or any known journey distance. You can read this from your trip meter or calculate it using a route planner. 2. Enter the fuel used in litres. The easiest way to measure this is to fill the tank completely, reset your trip meter, drive normally, then fill up again. The amount of fuel added at the second fill-up is the fuel used for that distance. 3. Enter the current fuel price per litre in GBP. You can find average UK fuel prices on the RAC Fuel Watch page or check the price displayed at your local forecourt. 4. Review your results. The headline figure is your imperial MPG, which you can compare directly with manufacturer WLTP ratings. The L/100km figure is useful if comparing with European vehicles. The cost per mile shows your actual running cost and is helpful for claiming mileage expenses or comparing with HMRC advisory rates. 5. For ongoing tracking, repeat this calculation each time you fill up. Over several fill-ups, you will build a reliable picture of your vehicle's real-world fuel economy across different driving conditions.

How It Works

The fuel economy calculation converts between UK imperial and metric measurements: Step 1: Convert litres to UK imperial gallons. Gallons used = fuel in litres / 4.546 The 4.546 figure is the exact number of litres in one UK imperial gallon. Do not confuse this with the US gallon (3.785 litres). Step 2: Calculate miles per gallon (imperial). MPG = distance in miles / gallons used This is the standard fuel economy measure used in the UK. Higher values mean better economy. Step 3: Convert distance to kilometres and calculate L/100km. Distance in km = distance in miles x 1.60934 L/100km = (fuel in litres / distance in km) x 100 L/100km is the standard metric measure used across Europe. Lower values mean better economy. Step 4: Calculate costs. Total fuel cost = fuel in litres x price per litre Cost per mile = total fuel cost / distance in miles Cost per km = total fuel cost / distance in km For example, driving 300 miles using 30 litres at GBP 1.45 per litre: Gallons = 30 / 4.546 = 6.60 gallons MPG = 300 / 6.60 = 45.5 mpg (imperial) Distance = 300 x 1.60934 = 482.8 km L/100km = (30 / 482.8) x 100 = 6.2 L/100km Total cost = 30 x 1.45 = GBP 43.50 Cost per mile = 43.50 / 300 = GBP 0.15