How to Calculate Fuel Cost for a Road Trip (and What Most People Miss)

The Basic Formula

Calculating fuel cost for a journey starts with one equation:

Fuel cost = (Distance ÷ Fuel efficiency) × Fuel price

For example: a 300-mile trip in a car that achieves 40 MPG, with fuel at 150p per litre, requires 300 ÷ 40 = 7.5 gallons = 34.1 litres. At 150p per litre, that's £51.15. Simple — but often wrong, because each input in that equation is less certain than it looks.

MPG vs L/100km: Getting the Units Right

The UK uses miles per gallon (MPG), the US uses miles per gallon (US gallons), and most of Europe uses litres per 100 kilometres (L/100km). These are not interchangeable.

One UK gallon is 4.546 litres; one US gallon is 3.785 litres. So a UK figure of 40 MPG equals roughly 33 US MPG. To convert L/100km to UK MPG: divide 282.5 by the L/100km figure. A car rated at 6 L/100km is approximately 47 MPG (UK).

When using online fuel cost calculators, always verify which unit system is in use. Mixing up UK and US gallons will make your estimate 20% off before you've left the driveway.

Why Real-World Fuel Economy Is Lower Than the Manufacturer Figure

Manufacturer fuel economy figures are measured on a standardised test cycle — the WLTP in Europe, EPA testing in the US. These tests are designed for consistency and comparability, not real-world accuracy. Research by the Transport Research Laboratory found that real-world fuel consumption is typically 15–20% worse than WLTP figures, and can be 25–30% worse in urban driving conditions.

The main factors that drag real-world efficiency below the test figure are: motorway speeds above 70 mph (where aerodynamic drag rises sharply), urban stop-start driving, air conditioning use in summer, carrying passengers or luggage, and cold engine starts. A car rated at 50 MPG might realistically achieve 38–43 MPG on a mixed road trip.

Highway vs City Economy

Fuel economy is not constant across different driving conditions. Most petrol and diesel cars are most efficient at steady speeds between 45–60 mph. Below that, inefficient combustion and stop-start losses dominate. Above 70 mph, aerodynamic drag — which increases with the square of speed — becomes the dominant force. Driving at 80 mph instead of 70 mph typically increases fuel consumption by 10–15%.

For a road trip calculation, if your route mixes motorway and local roads, use a blended figure: something 10–15% below the manufacturer's combined cycle rating is usually realistic for mixed driving, or 15–25% below if the route includes significant city traffic at either end.

Accounting for Fuel Price Variation on Long Routes

Fuel prices vary significantly by location. On a long UK route, prices at motorway service stations run 10–20p per litre higher than supermarket forecourts in the same region. In the US, fuel prices can vary by $0.50/gallon or more between states. For a 500-mile trip requiring 50+ litres, the difference between motorway and supermarket pricing could be £5–10.

For long trips, it's worth identifying one or two cheaper fuel stops along your route rather than filling up at the first available station. Apps like GasBuddy (US) or Petrol Prices (UK) show live prices by location.

Worked Example: A 500-Mile UK Road Trip

Route: London to Edinburgh (approximately 400 miles motorway, 100 miles mixed roads). Car: family hatchback, manufacturer WLTP combined rating of 48 MPG. Applying a 15% real-world adjustment gives an estimated 41 MPG. Fuel required: 500 ÷ 41 = 12.2 UK gallons = 55.5 litres.

Filling up at motorway services (average 158p/litre): £87.70. Filling up at planned supermarket stops (average 140p/litre): £77.70. The planning difference is £10 on this trip — not enormous, but worth five minutes of route planning.

Adding air conditioning use in summer adds roughly 5% to consumption — an extra 2.8 litres, about £4 more.

Tips for Reducing Fuel Cost on Long Drives

A few measures make a meaningful difference on long trips: maintaining steady speed with cruise control rather than repeated acceleration and braking; keeping tyre pressures at the correct level (under-inflation adds rolling resistance); removing roof boxes or bike racks when not in use (they add 10–15% fuel consumption at motorway speeds); and planning a departure time that avoids traffic jams, since idling in slow traffic is among the least efficient uses of fuel.

For more detail on each of these, our guide to improving fuel efficiency covers the evidence behind every technique — including which popular tips actually don't make any difference.