Planning a road trip is exciting — but the fuel bill can be a nasty surprise if you haven't estimated it properly. Whether you're comparing routes, deciding between vehicles, or budgeting for a European drive, understanding exactly how to calculate fuel costs gives you the numbers to make smart decisions. This guide walks through the formulas for UK and European fuel calculations, how to compare running costs across vehicle types, and tips for actually getting close to your theoretical fuel economy.
The Core Fuel Cost Formula
UK (MPG): Cost = (Distance in miles ÷ MPG) × Price per gallon
European (L/100km): Cost = (Distance in km ÷ 100) × Litres/100km × Price per litre
Note: 1 UK gallon = 4.546 litres
UK MPG Example: London to Edinburgh
Distance: 410 miles | Car fuel economy: 48 MPG | Petrol price: 147p/litre
First, convert price per litre to price per UK gallon:
Price per gallon = 147p × 4.546 = 667.3p = £6.673
Fuel cost = (410 ÷ 48) × £6.673 = 8.54 × £6.673 = £57.00
European L/100km Example: Paris to Nice
Distance: 930 km | Car: 6.5 L/100km | Diesel price in France: €1.58/litre
Litres needed = (930 ÷ 100) × 6.5 = 9.3 × 6.5 = 60.45 litres
Cost = 60.45 × €1.58 = €95.51
MPG vs L/100km Conversion
The two systems measure efficiency in opposite ways (higher MPG = more efficient; lower L/100km = more efficient).
L/100km = 282.48 ÷ MPG (UK)
MPG (UK) = 282.48 ÷ L/100km
Examples: 50 MPG = 5.65 L/100km | 40 MPG = 7.06 L/100km | 30 MPG = 9.42 L/100km
Note: UK MPG and US MPG are different — 1 UK gallon = 4.546 litres, 1 US gallon = 3.785 litres. A US 50 MPG vehicle = 42 UK MPG. Don't mix them up when looking at vehicle specs.
Real-World vs Official Fuel Economy
Manufacturers test fuel economy under controlled conditions. Real-world fuel economy typically runs 15–25% worse. Key factors:
- Motorway speed: Aerodynamic drag increases with speed squared. At 80 mph vs 70 mph, you use approximately 15% more fuel.
- Cold starts: Short trips with cold engines are far less efficient — fuel economy on a 2-mile trip might be half the official figure.
- Air conditioning: AC reduces economy by 5–15% in city driving.
- Roof box: A loaded roof box adds ~20% drag at motorway speeds, worsening economy by 10–15%.
- Tyre pressure: 20% under-inflation increases rolling resistance and worsens economy by 2–3%.
- Load: Every extra 50 kg reduces economy by approximately 1–2%.
For road trip planning, apply a 15% real-world correction: if official MPG is 55, use 47 for your calculation.
Comparing Vehicle Running Costs
| Vehicle type | Fuel efficiency | Cost per mile (2025) | London–Edinburgh |
|---|---|---|---|
| Petrol (average family car) | 42 MPG | ~14p/mile | £57.40 |
| Diesel (efficient saloon) | 55 MPG | ~11p/mile | £45.10 |
| Hybrid | 65 MPG equivalent | ~9.5p/mile | £38.95 |
| Electric (home charging, 4 miles/kWh) | 4 mi/kWh | ~6p/mile | £24.60 |
| Electric (motorway rapid charge) | 4 mi/kWh | ~17p/mile | £69.70 |
The EV cost story is complicated: home charging (typically 7p/kWh overnight) makes EVs very cheap per mile. But rapid charging (typically 68–79p/kWh) makes them more expensive than diesel for long motorway trips without home charging.
Multi-Stop Road Trip Calculator
For a trip with multiple legs, calculate each leg separately and sum:
Example UK road trip: Home (Birmingham) → Lake District (120 miles) → Edinburgh (155 miles) → Glasgow (46 miles) → Home (300 miles). Total: 621 miles in a 50 MPG diesel:
- Gallons used = 621 ÷ 50 = 12.42 gallons = 56.45 litres
- At 155p/litre diesel: 56.45 × £1.55 = £87.50
Ferry vs Drive: The Break-Even Calculation
Sometimes a longer drive to avoid a ferry pays off. To break even: compare total driving costs + time cost against ferry price + terminal transfer cost. If driving 120 miles extra costs £18 in fuel but the ferry costs £45, driving is cheaper — unless your time is worth more than £27 for the time saved.
Summary
UK: Fuel cost = (miles ÷ MPG) × price per gallon. Europe: (km ÷ 100) × L/100km × price per litre. Apply a 15% real-world correction to official MPG figures. EVs are cheapest on home charging; most expensive on motorway rapid chargers. Use our fuel cost calculator to model any trip, compare vehicle types, and factor in real-world adjustments.