MPG Calculator

Yes. Our MPG calculator lets you enter your registration plate to automatically estimate your car's MPG. We estimate this from your vehicle's CO2 emissions data (via public DVLA records), combined with MOT history mileage readings to estimate your annual driving distance. This auto-fills all calculators so you get personalised results instantly.

To calculate MPG (miles per gallon), divide the number of miles you've driven by the gallons of fuel used. For UK imperial gallons: MPG = Miles Driven ÷ Gallons Used. If you know litres, divide by 4.546 to convert to gallons first. Our MPG calculator does this instantly — just enter your distance, fuel spend, and the price per litre.

Use our Compare Cars calculator to enter the MPG of each vehicle, your annual mileage, and the current fuel price. It shows the fuel cost for each car and how much you could save per year by choosing the more efficient one. This is especially useful when deciding between two cars at a dealership or comparing your current car against an upgrade.

A real MPG calculator works out your actual, on-the-road fuel economy based on how much fuel you bought and how far you drove — rather than relying on manufacturer WLTP test figures. According to European Commission on-board monitoring data, real-world MPG is typically 15-25% lower than official figures due to driving conditions, traffic, weather, and driving style. Our Real MPG tab calculates this from your fuel receipts.

UK miles per gallon uses imperial gallons (4.546 litres), which is different from US gallons (3.785 litres). To calculate: fill your tank, note the mileage, drive until you refuel, then divide the miles driven by the litres used and multiply by 4.546. Or use our MPG calculator — enter your fuel spend, the price per litre, and the distance covered to get your exact UK MPG.

To convert UK MPG to L/100km, divide 282.48 by the MPG figure. For example: 40 MPG = 282.48 ÷ 40 = 7.06 L/100km. To convert L/100km to UK MPG, divide 282.48 by the L/100km figure. Use our Convert Units tab above for instant conversions.

At average UK petrol prices (around 140p per litre, per RAC Fuel Watch) and average fuel economy (38 MPG, per NimbleFins): 100 miles costs approximately £17. A very efficient car at 60 MPG would cost around £10.50, while a less efficient car at 30 MPG would cost around £21. Use our Journey Cost calculator for exact figures.

Yes. According to the DVSA, driving at 70mph uses up to 30% more fuel than 50mph. The AA reports that 80mph can use up to 25% more fuel than 70mph. The RAC confirms the most fuel-efficient speed is around 45-50mph in top gear. However, the biggest savings come from smooth acceleration and anticipating traffic to avoid unnecessary braking.

Yes. The US Department of Energy reports that air conditioning can reduce fuel economy by 5-25% depending on conditions. At low speeds (under 40mph), opening windows is more efficient. Research by Oak Ridge National Laboratory found that above around 60mph, the drag from open windows typically uses more fuel than the AC. Use AC sparingly at a moderate temperature.

A typical car uses 0.5-1 litre of fuel per hour when idling, depending on engine size. This equates to roughly 1-2p per minute. Research by Argonne National Laboratory found that idling for as little as 10 seconds uses more fuel than restarting the engine. Many modern cars have automatic stop-start systems for this reason.

According to Autocar, diesel engines are typically 20-35% more fuel efficient than petrol engines of equivalent power, due to higher compression ratios and fuel energy density. However, diesel costs more per litre and diesel cars have higher purchase prices. Evans Halshaw and Honest John recommend diesel mainly for high-mileage drivers (over 12,000 miles per year), particularly those doing motorway driving.