fuel economy calculator uk

How to use this fuel economy calculator (UK)

This fuel economy calculator UK tool is designed for drivers who track mileage in miles and buy fuel in litres. It converts your numbers into UK MPG (miles per imperial gallon), plus other useful formats such as litres per 100km. If you also enter fuel price, you get a practical cost breakdown that helps with budgeting and trip planning.

To get the most accurate result, use the full-tank method: fill your tank, drive as normal, then fill up again and record the litres and miles covered between fills. One short journey can be noisy, but tracking several tanks gives a realistic long-term average.

What does the calculator show?

• MPG (UK): miles travelled per imperial gallon of fuel.
• Litres per 100km: commonly used in Europe for efficiency comparison.
• Km per litre: another easy efficiency metric.
• Cost per mile: useful for commuting and reimbursement planning.
• Trip fuel cost: direct fuel spend for that distance.
• Estimated annual fuel usage and cost: if annual mileage is provided.

UK MPG vs US MPG: why the number changes

A common source of confusion is that UK and US gallons are not the same size. In the UK, one imperial gallon equals 4.54609 litres. In the US, one gallon is only 3.785 litres. That means a car can show a higher MPG value in UK units than in US units even when real fuel use is identical.

This page uses UK imperial MPG, which is the expected standard for most UK drivers and motoring publications.

Fuel economy formulas used

1) MPG (UK)

MPG (UK) = miles ÷ (litres ÷ 4.54609)

2) Litres per 100km

L/100km = (litres ÷ kilometres) × 100, where kilometres = miles × 1.609344

3) Cost per mile

Cost per mile = (litres × price per litre) ÷ miles

Example calculation

Suppose you drove 320 miles and used 34.8 litres of petrol. At a fuel price of 147.9 pence per litre:

• MPG (UK) is about 41.8 mpg
• L/100km is about 8.8
• Cost per mile is roughly 16.1p
• Trip fuel cost is approximately £51.47

These numbers are ideal for comparing vehicles, spotting changes in driving efficiency, and setting a realistic monthly transport budget.

How to improve fuel economy in real UK driving

Driving style changes

• Accelerate smoothly and avoid unnecessary hard braking.
• Use the highest practical gear without labouring the engine.
• Maintain steady speed on A-roads and motorways where safe.
• Anticipate traffic to reduce stop-start waste.

Vehicle setup and maintenance

• Keep tyres inflated to manufacturer recommendation.
• Remove unnecessary weight from boot and cabin.
• Use correct oil grade and service intervals.
• Fix warning lights quickly (e.g., oxygen sensor, DPF issues, injector faults).

Journey planning

• Combine short errands into one longer trip.
• Avoid peak congestion when possible.
• Choose routes with fewer full stops if travel time is similar.

Common mistakes when calculating mpg

• Mixing gallons and litres: this creates incorrect MPG values.
• Using dashboard estimate only: trip computers can be optimistic or pessimistic.
• Measuring one very short trip: cold starts distort results.
• Forgetting fuel price units: UK forecourts display pence per litre, not pounds.

When to use this calculator

This fuel economy calculator UK page is useful for:

• Comparing two cars before purchase.
• Estimating commuting costs after a job or office move.
• Monitoring efficiency decline over time.
• Checking whether a tune-up improved consumption.
• Building a yearly household transport budget.

Final thoughts

Fuel costs are one of the biggest variable expenses for UK drivers, but they are also highly measurable. By logging miles, litres, and price per litre, you can make data-driven decisions about route choice, maintenance, and even whether changing vehicle type would save money long term. Use this calculator regularly and watch trends, not just one-off numbers.