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How to work out fuel cost
CheckFuelPrices Editorial
Expert Written • 4 industry sources
To work out fuel cost, divide your journey distance (in miles) by your car's MPG to get litres needed, then multiply by the price per litre. The formula is: (Miles ÷ MPG) × 4.546 × Pence per litre ÷ 100 = cost in pounds.
The Fuel Cost Formula
Step-by-step calculation:
Take your journey distance in miles, divide by your car's MPG to get gallons used, multiply by 4.546 to convert to litres, then multiply by the pump price in pence and divide by 100 to get pounds.
Worked example:
A 100-mile trip in a car doing 40 MPG, with fuel at 140p per litre: (100 ÷ 40) × 4.546 × 140 ÷ 100 = £15.91.
Why 4.546?:
UK MPG figures are based on imperial gallons, which equal 4.546 litres. Using 3.785 (US gallon) is a common mistake that will underestimate your fuel cost.
Getting the Numbers Right
Your real-world MPG:
Manufacturer MPG figures are typically 20–30% higher than real-world consumption. Use your trip computer reading or calculate from your last few fill-ups for an accurate figure.
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Current fuel prices vary widely:
UK petrol and diesel prices shift weekly and differ significantly between stations and regions. Using an outdated price will skew your estimate.
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Petrol vs diesel costs differ:
Diesel typically offers better MPG but has historically cost more per litre, so the cost-per-mile comparison between fuel types is not always obvious without calculating both.
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Factors That Change Your Fuel Cost
Motorway vs town driving:
Stop-start urban driving can cut your effective MPG by 30–40% compared to a steady motorway cruise, significantly increasing cost per mile.
Vehicle load and speed:
Carrying extra weight and driving above 70 mph both reduce fuel efficiency noticeably. Cutting motorway speed from 80 to 70 mph can improve MPG by around 10%.
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Air conditioning:
Running air conditioning at low speeds increases fuel consumption by up to 10%, adding meaningfully to short-trip costs.
Getting the Cheapest Price Per Litre
Price is the easiest variable to control:
You cannot always change your MPG or journey length, but you can choose where you fill up. Even a 5p per litre saving on a 50-litre fill is £2.50 back in your pocket.
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Compare live prices before you fill up:
CheckFuelPrices shows live petrol and diesel prices at 4,000+ UK stations, updated every 30 minutes from CMA government data, so you can plug the cheapest local price directly into your calculation.
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Sources
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