Road trips are one of the great pleasures — but fuel costs can be a nasty surprise. With this guide, you can estimate your fuel bill accurately before you leave the driveway.
The Fuel Cost Formula
Fuel cost = (Distance / Fuel efficiency) × Price per litre (or gallon)
In metric (km and litres):
Cost = (Distance in km / Fuel economy in km/L) × Price per litre
In imperial (miles and UK gallons):
Cost = (Miles / MPG) × Price per gallon
In US (miles and US gallons):
Cost = (Miles / MPG) × Price per gallon
Worked Example: UK Road Trip
Journey: London to Edinburgh, 640 km Your car: 50 mpg (UK gallons), equivalent to about 17.7 km/L Fuel price: £1.55 per litre
Convert MPG to km/L:
MPG (UK) × 0.354 = km/L
50 × 0.354 = 17.7 km/L
Fuel needed:
640 km / 17.7 km/L = 36.2 litres
Cost:
36.2 × £1.55 = £56.11 one way
Return trip: £112.22
Worked Example: US Road Trip
Journey: New York to Boston, 340 miles Car: 32 mpg (US) Gas price: $3.60 per gallon
Fuel needed = 340 miles / 32 mpg = 10.6 gallons
Cost = 10.6 × $3.60 = $38.18 one way
Unit Conversions
| From | To | Multiply by |
|---|---|---|
| MPG (UK) | L/100km | 282.5 ÷ MPG |
| MPG (US) | L/100km | 235.2 ÷ MPG |
| L/100km | MPG (UK) | 282.5 ÷ L/100km |
| L/100km | MPG (US) | 235.2 ÷ L/100km |
| UK gallons | Litres | × 4.546 |
| US gallons | Litres | × 3.785 |
| Miles | Kilometres | × 1.609 |
Fuel Economy by Car Type
| Vehicle type | Typical fuel economy |
|---|---|
| City car (petrol) | 50–65 mpg / 5–6 L/100km |
| Family hatchback (petrol) | 40–55 mpg / 5–7 L/100km |
| SUV (petrol) | 30–40 mpg / 7–9 L/100km |
| Large SUV/4WD | 20–30 mpg / 9–14 L/100km |
| Diesel (equivalent) | Add ~15–20% efficiency |
| Hybrid | Add ~30–50% efficiency |
| EV | 3–4 miles per kWh (see below) |
Electric Vehicles: Cost per Mile
For EVs, calculate in cost per mile or cost per km:
Cost = (Distance / miles per kWh) × electricity rate
Example: EV doing 4 miles/kWh, charging at home at 28p/kWh, driving 340 miles:
Energy needed = 340 / 4 = 85 kWh
Cost = 85 × £0.28 = £23.80
Compared to a 40 mpg petrol car at £1.55/L:
Fuel = 340 miles / 40 mpg = 8.5 UK gallons = 38.6 litres
Cost = 38.6 × £1.55 = £59.83
EV saves £36.03 on this journey (home charging vs petrol).
Motorway Charging Costs
EV charging at motorway rapid chargers (typically 60–80p/kWh) changes the calculation:
Cost = 85 kWh × £0.70 = £59.50
This nearly matches petrol costs — home charging is essential for EV economics.
Planning a Multi-Stop Trip
Break the journey into legs and calculate each:
| Leg | Distance | Fuel |
|---|---|---|
| London → Birmingham | 183 km | 10.3 L |
| Birmingham → Manchester | 130 km | 7.3 L |
| Manchester → Edinburgh | 340 km | 19.2 L |
| Total | 653 km | 36.8 L |
Real-World vs Advertised Fuel Economy
Manufacturer fuel economy figures are tested under controlled lab conditions. Real-world fuel economy is typically:
- Motorway driving: 85–95% of advertised
- Mixed driving: 75–85% of advertised
- City driving: 60–75% of advertised
- Cold weather: 10–20% worse
- Roof box/heavy load: 10–25% worse
- Air conditioning in summer: 5–15% worse
Safe rule: Use 85% of your car's official mpg figure for highway trips, 70–75% for mixed routes.
Quick Cost Reference (£1.55/L petrol)
| Distance | 40 mpg car | 50 mpg car | 60 mpg car |
|---|---|---|---|
| 100 miles | £17.80 | £14.24 | £11.87 |
| 200 miles | £35.61 | £28.48 | £23.74 |
| 300 miles | £53.41 | £42.73 | £35.61 |
| 500 miles | £89.02 | £71.21 | £59.34 |
| 1,000 miles | £178.04 | £142.42 | £118.69 |