Understanding your fuel consumption helps you budget and compare vehicles. This guide shows how to calculate MPG, cost per mile, and annual fuel costs.
MPG Calculation Formula
Or using litres: MPG = Miles driven × 4.546 ÷ Litres used
Example Calculation
Miles driven: 400
Fuel used: 45 litres
MPG = 400 × 4.546 ÷ 45 = 40.4 mpg
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Cost Per Mile
Or: Cost per mile = Fuel price per litre × Litres per mile
Cost Per Mile by MPG (at 145p/litre)
| MPG | Litres/100km | Cost/Mile | 10,000 miles/year |
|---|---|---|---|
| 30 mpg | 9.4 L/100km | 10.6p | £1,060 |
| 40 mpg | 7.1 L/100km | 8.0p | £800 |
| 50 mpg | 5.6 L/100km | 6.4p | £640 |
| 60 mpg | 4.7 L/100km | 5.3p | £530 |
| 70 mpg | 4.0 L/100km | 4.6p | £460 |
UK Fuel Prices (Late 2024)
| Fuel Type | Average Price |
|---|---|
| Unleaded petrol | ~142-148p/litre |
| Diesel | ~150-156p/litre |
| Super unleaded | ~155-165p/litre |
| Premium diesel | ~160-170p/litre |
Petrol vs Diesel vs Electric
| Factor | Petrol | Diesel | Electric |
|---|---|---|---|
| Fuel cost/mile | 8-12p | 7-11p | 4-6p |
| Annual fuel (10k mi) | £800-1,200 | £700-1,100 | £400-600 |
| Best for | Short trips | Motorway miles | All use cases |
| VED (first year) | Variable | Variable | £0 |
Improving Fuel Economy
- Proper tyre pressure: Underinflated tyres waste fuel
- Smooth driving: Avoid harsh acceleration/braking
- Remove excess weight: Clear out the boot
- Reduce drag: Remove roof racks when not in use
- Use cruise control: Maintains constant speed on motorways
- Plan routes: Avoid congestion and stop-start traffic
- Regular service: Clean air filters and fresh oil help efficiency
How Fuel Consumption Calculations Work: The Methodology
Fuel consumption in the UK is traditionally measured in miles per gallon (mpg), using the imperial gallon (4.546 litres), which is larger than the US gallon (3.785 litres). This distinction is important because a car rated at 40 mpg in the UK would appear to achieve only 33 mpg using US measurements. Always ensure you are comparing like with like when reading fuel economy data from different sources.
The fundamental MPG calculation requires two pieces of data: the distance driven and the fuel consumed. The brim-to-brim method is the most accurate approach. Fill your tank completely, reset your trip counter, drive normally until you need to refuel, then fill the tank completely again. Record the miles driven and the litres added at the second fill. Then apply the formula: MPG = Miles driven x 4.546 / Litres used.
For cost per mile calculations, you divide the fuel cost per litre by the number of litres per mile your car uses. Alternatively, you can use: Cost per mile = (Fuel price per litre x 100) / (MPG x 4.546). This gives you the pence per mile figure. Multiplied by your expected annual mileage, this yields your total annual fuel cost.
European fuel economy figures use litres per 100 kilometres (L/100km), which is the inverse of MPG. To convert: L/100km = 282.48 / MPG (imperial). A car doing 40 mpg uses approximately 7.06 L/100km. The lower the L/100km figure, the more efficient the vehicle. Since 2019, all new cars sold in the UK display WLTP (Worldwide Harmonised Light Vehicle Test Procedure) consumption figures, which are more representative of real-world driving than the older NEDC figures they replaced.
UK-Specific Context: Fuel Costs and Regulations
UK fuel prices are among the highest in Europe, largely due to fuel duty (currently 52.95p per litre, after the temporary 5p cut introduced in March 2022 was extended) and 20% VAT applied on top. This means that over 50% of the pump price goes to the government in taxes. The RAC Foundation and AA regularly publish average UK fuel price data, which you can use to track trends and plan your spending.
Vehicle Excise Duty (VED), commonly known as road tax, is calculated partly based on CO2 emissions, which correlate directly with fuel consumption. For cars first registered from April 2017, first-year VED ranges from £0 for zero-emission vehicles to over £2,745 for the most polluting models. From the second year onwards, most petrol and diesel cars pay a flat rate of £190 per year (2025/26), with an additional surcharge for vehicles with a list price over £40,000.
The UK government's commitment to net zero by 2050 has significant implications for fuel costs and vehicle choice. The ban on the sale of new petrol and diesel cars was originally set for 2030 but has been pushed back to 2035. However, the market is shifting rapidly, with battery electric vehicles (BEVs) now accounting for over 20% of new car sales. For those comparing options, the electricity cost to charge an EV (approximately 4-6p per mile on a home charger at off-peak rates) is significantly lower than the 8-12p per mile for petrol or diesel.
Clean Air Zones (CAZs) and Ultra Low Emission Zones (ULEZ) in UK cities also factor into total running costs. London's ULEZ charges £12.50 per day for non-compliant vehicles, while cities such as Birmingham, Bristol, and Bradford have their own schemes targeting older, more polluting vehicles. These charges effectively increase the per-mile cost of driving a fuel-inefficient vehicle in urban areas.
Worked Examples: Real-World Fuel Cost Calculations
Example 1: Weekly Commuter Fuel Cost
You drive 30 miles each way to work, 5 days per week. Your car achieves 45 mpg. Petrol costs 145p/litre.
Weekly miles: 30 x 2 x 5 = 300 miles
Litres used: 300 / (45 / 4.546) = 300 / 9.88 = 30.36 litres
Weekly cost: 30.36 x £1.45 = £44.02 per week
Annual commuting fuel: £44.02 x 48 working weeks = £2,113 per year
Example 2: Petrol vs Electric Comparison
Annual mileage: 10,000 miles. Petrol car at 40 mpg vs EV at 3.5 miles/kWh.
Petrol cost: 10,000 / (40/4.546) x £1.45 = £1,649 per year
EV cost (home charging at 7.5p/kWh off-peak): (10,000/3.5) x £0.075 = £214 per year
Annual fuel saving with EV: £1,435
Example 3: Holiday Road Trip Budget
Driving from London to Edinburgh (approximately 400 miles). Diesel car achieving 55 mpg. Diesel at 152p/litre.
Litres needed: 400 / (55/4.546) = 400 / 12.10 = 33.1 litres
One-way fuel cost: 33.1 x £1.52 = £50.31
Return trip: Approximately £100.62
Common Mistakes and Tips
Frequently Asked Questions
How accurate are WLTP fuel consumption figures?
WLTP figures are more realistic than the older NEDC figures they replaced, but most drivers still find real-world consumption 10-20% higher than the official figure. The WLTP test includes a wider range of speeds and driving conditions, but it still cannot perfectly replicate individual driving styles, weather conditions, and terrain. Use the official figure as a comparative benchmark, then expect real-world performance to be somewhat lower.
Is it cheaper to fill up with E10 or E5 petrol in the UK?
E10 petrol (containing up to 10% ethanol) became the standard UK pump fuel in September 2021 and is typically 5-8p per litre cheaper than E5 (super unleaded). However, E10 contains slightly less energy per litre, reducing fuel economy by approximately 1-2%. For most drivers, the lower pump price of E10 outweighs the marginal efficiency loss, making it the more cost-effective choice. Older vehicles manufactured before 2002 may not be compatible with E10 -- check the government's online checker.
Can I claim HMRC mileage for my daily commute?
No. HMRC mileage allowance payments (MAPs) only apply to business miles, not ordinary commuting. Business miles include travel between different work locations, visiting clients, or attending meetings away from your normal workplace. If you use your personal vehicle for business travel, your employer can reimburse you at up to 45p per mile for the first 10,000 miles and 25p per mile thereafter without triggering a tax liability.
How much can I save by driving at 60 mph instead of 70 mph?
According to the Department for Transport, reducing your motorway speed from 70 mph to 60 mph can improve fuel efficiency by approximately 10-15%. For a car achieving 40 mpg at 70 mph, this could increase to 44-46 mpg at 60 mph. Over 10,000 motorway miles, this equates to a saving of roughly £100-180 per year at current fuel prices. The trade-off is additional journey time -- about 15 minutes extra per 100 miles.
HMRC Mileage Rates 2025/26
| Vehicle | First 10,000 miles | Over 10,000 miles |
|---|---|---|
| Car/Van | 45p per mile | 25p per mile |
| Motorcycle | 24p per mile | 24p per mile |
| Bicycle | 20p per mile | 20p per mile |
Practical Tips for Reducing UK Fuel Costs
With fuel prices in the UK consistently among the highest in Europe, every practical measure to reduce consumption translates directly into meaningful savings for drivers. Here are UK-specific strategies that go beyond the standard fuel-saving advice.
Use supermarket fuel price comparison tools. UK fuel prices can vary by as much as 10 to 15 pence per litre between stations in the same town. Websites like PetrolPrices.com and the RAC Fuel Watch service allow you to compare prices at nearby stations by postcode. Supermarket forecourts at Asda, Tesco, Sainsbury's, and Morrisons are typically 3 to 7 pence per litre cheaper than branded stations like Shell and BP. For a car with a 50-litre tank, saving 5 pence per litre saves 2.50 pounds per fill-up, which adds up to approximately 100 to 130 pounds per year for the average UK driver filling up weekly.
Consider fuel loyalty cards and cashback. Several UK retailers offer fuel savings through loyalty schemes. The Tesco Clubcard provides points on fuel purchases that can be converted into money-off vouchers. The Nectar card earns points at Sainsbury's and Esso stations. Shell Go+ offers savings at Shell forecourts. Additionally, cashback credit cards such as the American Express Platinum Everyday card or Santander All in One card can provide 0.5 to 1 percent cashback on fuel purchases, adding a further small but cumulative saving over the course of a year.
Monitor tyre pressure monthly. The RAC estimates that under-inflated tyres increase fuel consumption by up to 3 percent and reduce tyre lifespan. Most UK petrol stations offer free or low-cost tyre pressure checks. The correct pressures for your vehicle are usually printed on a sticker inside the driver's door frame or in the owner's manual. Check pressures when tyres are cold (before driving) for the most accurate reading, and remember that pressures may need to be higher when carrying heavy loads or towing, as specified in your vehicle handbook.
Emma Thompson
Senior Content Editor
Emma is a senior content editor with a background in financial journalism. She specialises in making UK regulations and calculator tools understandable for consumers, working closely with qualified professionals to ensure accuracy.