Calculate gas mileage, fuel consumption, and unit-converted efficiency results.
This tool provides estimates for informational purposes only. It is not a substitute for professional advice. Individual results vary based on your inputs and assumptions, so review important decisions with a qualified professional.
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Knowing your real-world gas mileage (called fuel economy in the UK) is one of the most valuable pieces of information a driver can have. Manufacturer-quoted figures are often measured under idealised laboratory conditions, frequently overstating real-world performance. This guide explains exactly how to calculate your MPG accurately, what the US and UK standards mean, why they differ, and how to improve your fuel economy in practical terms.
The core formula for calculating miles per gallon is simple:
MPG = Miles Driven / Gallons Used
Using the fill-up method (the most accurate real-world approach):
UK drivers using litres: convert litres to imperial gallons first by dividing by 4.546, then divide miles by that figure. Alternatively: MPG (imperial) = miles / (litres / 4.546).
The most important thing to understand about MPG is that US MPG and UK MPG are not the same — even when measuring the exact same car on the exact same journey.
| Standard | Gallon Size | Example: 45 MPG = |
|---|---|---|
| US MPG | 3.785 litres (US gallon) | 19.1 km/L or 5.24 L/100km |
| UK MPG (imperial) | 4.546 litres (imperial gallon) | 15.9 km/L or 6.29 L/100km |
| Conversion | UK MPG x 0.8327 = US MPG | 45 UK MPG = 37.5 US MPG |
A UK car advertised at 60 MPG is only achieving 50 US MPG. This difference of nearly 20% means you should never directly compare UK and US fuel economy figures. Always convert first using the 0.8327 multiplier (UK to US) or 1.201 multiplier (US to UK).
Most European countries — and increasingly UK comparisons post-WLTP adoption — express fuel economy as litres per 100 kilometres (L/100km). Unlike MPG where higher is better, with L/100km lower is better.
| L/100km | UK MPG (imperial) | US MPG | Vehicle Type |
|---|---|---|---|
| 3.5 | 80.7 | 67.2 | Exceptional — PHEV, small hybrid |
| 5.0 | 56.5 | 47.1 | Efficient small diesel or hybrid |
| 7.0 | 40.4 | 33.6 | Average petrol family car |
| 10.0 | 28.2 | 23.5 | Large SUV or performance car |
| 15.0 | 18.8 | 15.7 | Supercar or large truck |
The US Environmental Protection Agency (EPA) requires all new cars sold in the US to display fuel economy ratings on a standardised window sticker. The EPA tests vehicles under laboratory conditions across two drive cycles:
Real-world MPG frequently differs from EPA estimates. The EPA itself notes that actual results may vary based on how, where, and when you drive, cargo weight, optional features, and weather. US drivers typically achieve 70–80% of the EPA highway rating in mixed real-world driving.
From September 2018, the UK and EU adopted the WLTP (Worldwide Harmonised Light Vehicle Test Procedure) to replace the older NEDC test. WLTP is significantly more realistic than the old test:
WLTP figures are still typically 10–20% better than real-world results but are much more realistic than the old NEDC figures, which were routinely 30–40% better than real-world fuel economy. When comparing older UK car reviews to post-2018 cars, note which test cycle was used.
| Factor | Typical Impact on MPG | Notes |
|---|---|---|
| Tyre pressure (10 PSI low) | -1 to -2% | Check monthly; increases rolling resistance |
| Air conditioning | -5 to -15% | Greatest at low speed; at 70 mph, open windows cost more |
| Speed increase 60 to 70 mph | -15 to -25% | Aerodynamic drag increases with square of speed |
| Extra 100 lbs / 45 kg cargo | -1 to -2% | Roof box adds weight plus significant drag |
| Cold engine (short trips) | -20 to -40% | Engine runs rich until warm; catalytic converter inactive |
| Aggressive driving style | -15 to -30% | Biggest single controllable factor |
| Car Class | Real-World MPG (UK imperial) | Examples |
|---|---|---|
| City car | 40–55 MPG | MINI, Fiat 500, VW Polo |
| Small family (C-segment) | 35–50 MPG | Ford Focus, VW Golf, Vauxhall Astra |
| Large family (D-segment) | 30–45 MPG | Ford Mondeo, Vauxhall Insignia |
| SUV / crossover | 25–40 MPG | Ford Kuga, Nissan Qashqai |
| Full hybrid | 50–65 MPG | Toyota Prius, Honda Jazz Hybrid |
Use the fill-up method: fill your tank completely, reset the trip odometer, drive normally, then fill up again. Divide the miles driven by the gallons (or litres converted to gallons) you added. This gives your true real-world MPG, which is typically 10–20% lower than manufacturer figures.
No. UK MPG uses imperial gallons (4.546 litres) while US MPG uses US gallons (3.785 litres). A UK imperial gallon is about 20% larger, so UK MPG figures are about 20% higher than US MPG for the same vehicle. To convert UK MPG to US MPG, multiply by 0.8327.
WLTP (Worldwide Harmonised Light Vehicle Test Procedure) replaced the old NEDC test for UK and EU cars from 2018. WLTP produces more realistic fuel economy figures that are closer to actual real-world driving, though still typically 10–20% better than what most drivers achieve. Old NEDC figures were routinely 30–40% better than real-world results.
For a petrol family hatchback, 40–50 MPG (imperial) is good real-world performance. Diesel equivalents typically achieve 45–60 MPG. Hybrids reach 50–70 MPG. Anything below 30 MPG for a standard petrol car is poor economy. Large petrol SUVs commonly return 25–35 MPG in real-world UK driving.
MPGe (miles per gallon equivalent) is the US EPA's way of comparing EV efficiency to petrol cars. It is calculated using 33.7 kWh as the energy equivalent of one US gallon of gasoline. A Tesla Model 3 achieves around 132 MPGe combined — roughly 3–4 times more efficient than a typical petrol car. The UK equivalent measure is miles per kWh (typically 3–4.5 mi/kWh for modern EVs).
Cold weather reduces MPG by 15–25% for petrol cars and up to 40% for EVs. Causes include: cold engine running rich on fuel until warm, increased rolling resistance from cold tyres and thicker engine oil, heater and defroster electrical loads, and greater aerodynamic drag from cold dense air. Short trips in winter barely let the engine reach operating temperature, causing the greatest fuel economy penalties.
Key improvements: maintain correct tyre pressures (check monthly), drive smoothly with gentle acceleration and anticipatory braking, reduce speed (70 mph uses 25% more fuel than 60 mph), remove unused roof racks, use air conditioning sparingly at low speeds, service the car regularly (dirty air filter and worn spark plugs both reduce economy), and avoid excessive idling.
According to the US DOT and EPA, the average fuel economy of new cars and light trucks sold in the US was approximately 27–28 MPG (US) combined in recent years. SUVs and trucks (which make up the majority of US new vehicle sales) average around 20–25 MPG, while passenger cars average 30–40 MPG. The CAFE fleet average standard targets 49 MPG by 2026 for passenger cars.