How to Calculate UK Fuel Costs: Complete 2026 Guide

Knowing what a journey will cost in fuel before you set off lets you budget accurately, decide whether driving is cheaper than the train, and plan fuel stops on longer routes. This guide walks through exactly how to calculate fuel costs for any UK journey, explains the difference between MPG and L/100km, gives real worked examples for common routes, and covers practical ways to cut your fuel bill by up to 20% without changing how far you drive.

Understanding Fuel Consumption: MPG, L/100km, and How UK Figures Differ from Reality

Fuel consumption in the UK is most commonly expressed in miles per gallon (MPG) — the number of miles your vehicle travels on one imperial gallon of fuel (4.546 litres). The higher the MPG figure, the more efficient the vehicle.

European specifications often use litres per 100 kilometres (L/100km), which works in the opposite direction — a lower figure means better fuel economy. Some newer UK vehicles display both.

Converting between MPG and L/100km

To convert MPG to L/100km: divide 282.5 by the MPG figure.
To convert L/100km to MPG: divide 282.5 by the L/100km figure.

Example: A car with a published fuel economy of 50 MPG equals 282.5 ÷ 50 = 5.65 L/100km.

Why official MPG figures are misleading

The official fuel economy figures on new car specifications are generated under WLTP (Worldwide Harmonised Light Vehicles Test Procedure) laboratory conditions — standardised test cycles that do not reflect real-world driving. In practice, most drivers achieve 10–25% below the official MPG figure, depending on:

• Driving style — aggressive acceleration and hard braking reduce economy significantly.
• Speed — fuel consumption rises sharply above 60 mph due to air resistance.
• Load — a full car with luggage uses more fuel than an empty one.
• Climate — air conditioning in summer and heating in winter both reduce efficiency. Cold weather also thickens engine oil and increases rolling resistance.
• Traffic — stop-start urban driving is much less efficient than steady motorway cruising.
• Tyre condition — under-inflated or worn tyres increase rolling resistance.

For fuel cost calculations, use your actual observed MPG if you know it (many cars display a trip computer with fuel economy data), or take the official figure and apply a 15–20% reduction as a realistic estimate.

Average MPG by Vehicle Type (Real-World Estimates)

These are real-world estimates. Your actual consumption will depend on driving style, load, and conditions.

The Fuel Cost Formula: Step by Step

The basic formula for calculating fuel cost in the UK (where fuel is sold by the litre and consumption is measured in MPG) is:

Breaking this down:

1. (Distance ÷ MPG) gives you how many imperial gallons you will use.
2. × 4.546 converts imperial gallons to litres (since fuel is priced in litres at the pump).
3. × Price per litre gives you the total cost in pence, which you divide by 100 for pounds.

Alternatively, if you know your fuel consumption in L/100km:

Worked Examples: Common UK Journeys

Example 1: London to Birmingham (120 miles)

Vehicle: Medium petrol car, 40 MPG real-world
Fuel price: £1.45/litre (petrol)

Calculation:

• Gallons used: 120 ÷ 40 = 3 gallons
• Litres used: 3 × 4.546 = 13.64 litres
• Cost: 13.64 × £1.45 = £19.78

Add the M6 Toll if you use it (£6.90 for cars) = total journey cost approximately £26.68.

Example 2: London to Manchester (200 miles)

Vehicle: Medium diesel car, 48 MPG real-world
Fuel price: £1.50/litre (diesel)

Calculation:

• Gallons used: 200 ÷ 48 = 4.17 gallons
• Litres used: 4.17 × 4.546 = 18.96 litres
• Cost: 18.96 × £1.50 = £28.44

Example 3: Edinburgh to Glasgow (46 miles)

Vehicle: Small petrol car, 48 MPG real-world
Fuel price: £1.47/litre (petrol)

Calculation:

• Gallons used: 46 ÷ 48 = 0.96 gallons
• Litres used: 0.96 × 4.546 = 4.36 litres
• Cost: 4.36 × £1.47 = £6.41

UK Fuel Prices in 2026: What to Expect

UK petrol and diesel prices are set daily by fuel retailers and fluctuate based on crude oil prices, refinery costs, currency exchange rates (oil is priced in US dollars), and government duty. The RAC and AA publish daily fuel price data.

As of early 2026, UK pump prices are approximately:

Fuel Costs vs. EV Charging Costs

Electric vehicles are becoming increasingly common, and calculating the cost per mile for an EV requires a different approach. Instead of MPG, EVs use kWh per 100 miles (or miles per kWh).

For comparison, a petrol car achieving 40 MPG at £1.43/litre costs approximately 16p per mile in fuel. An EV charged at home costs around 6–10p per mile. The savings are clear for home charging — but rapid charging on long motorway journeys brings EV costs closer to petrol costs.

Practical Ways to Reduce Your Fuel Bill

Small changes in driving behaviour and vehicle maintenance can reduce fuel consumption by 10–20% without sacrificing journey time significantly.

Driving technique

• Accelerate gently: Hard acceleration from rest is the single biggest source of fuel waste in urban driving. Build speed gradually, particularly from traffic lights and roundabouts.
• Read the road ahead: Anticipating traffic lights, junctions, and slowing traffic allows you to ease off the accelerator and coast — converting your speed back to fuel savings rather than heat in your brakes.
• Use a higher gear earlier: Modern engines are efficient at low RPM in high gears. Change up at around 2,000–2,500 RPM for petrol engines, 1,500–2,000 RPM for diesel. Your car's gear change indicator (if fitted) will prompt you.
• Use cruise control on motorways: Cruise control maintains a constant speed much more effectively than most drivers can manually, eliminating the unconscious speed fluctuations that waste fuel.
• Reduce motorway speed: Driving at 60 mph instead of 70 mph uses roughly 9% less fuel. At 80 mph, consumption is approximately 25% higher than at 70 mph. Even a 5 mph reduction on a long journey can make a meaningful difference.

Vehicle preparation

• Maintain correct tyre pressure: Under-inflated tyres increase rolling resistance. The RAC estimates that correctly inflated tyres can improve fuel economy by up to 2%. Check pressures monthly and before long journeys (cold, not after driving).
• Remove unnecessary weight: Every 50 kg of extra weight increases fuel consumption by approximately 1–2%. Remove items you do not need from the boot — roof bars, sports equipment, tools.
• Remove roof boxes and carriers when not in use: A roof box increases drag significantly and can reduce fuel economy by 10–15% at motorway speeds. If you only use it for holidays, take it off between trips.
• Keep the engine serviced: A poorly maintained engine — old spark plugs, dirty air filter, old engine oil — burns more fuel. Regular servicing typically pays for itself in fuel savings over time.
• Turn off air conditioning when not needed: Air conditioning can increase fuel consumption by up to 10% in city driving. At motorway speeds, it is often more efficient to use the air conditioning than to open windows (which increases drag).

Buying fuel smartly

• Avoid motorway services: Motorway service station fuel typically costs 10–20p/litre more than nearby supermarket stations. Planning to fill up before joining the motorway saves money.
• Use supermarket loyalty schemes: Tesco, Morrisons, and Sainsbury's all offer points on fuel purchases. Morrisons More and Tesco Clubcard schemes can generate meaningful savings for regular users.
• Use a cashback credit card: Some credit cards offer 1–2% cashback on fuel purchases. On a household spending £1,500/year on fuel, that is £15–30 in annual cashback — not enormous, but free money nonetheless.
• Fill up in the morning: Fuel expands slightly in warmer temperatures. Filling up early in the day when the fuel in underground tanks is cooler means you get marginally more fuel per litre. This is a minor effect in the UK climate but is measurable.

Quick Fuel Cost Calculator Reference

Use this table to quickly estimate fuel costs for typical journey distances at common MPG values and a petrol price of £1.43/litre:

Based on petrol at £1.43/litre. For diesel at £1.49/litre, multiply figures by 1.04.