How to Work Out the Fuel Cost of Any Road Trip

Before a long drive, most people want to know two things: how long it will take and how much it will cost. The time is easy to find on any route planner. The cost takes a little arithmetic, but once you know the formula you can work it out for any trip in under a minute.

The basic formula

Fuel cost depends on three numbers: how far you drive, how much fuel your car uses, and what fuel costs per litre. Most cars list their consumption in litres per 100 kilometres (L/100 km), so the formula looks like this:

That's all there is to it. The rest of this guide covers where to find each number and how to make the estimate closer to what you'll actually pay.

A worked example

Say you're driving 140 km each way in a car that uses 8.5 L/100 km, and fuel costs 2.60 per litre in your local currency.

1. Litres for one way: 140 × 8.5 ÷ 100 = 11.9 litres
2. Cost one way: 11.9 × 2.60 = 30.94
3. Round trip: 30.94 × 2 = 61.88

If four people share the car, that's about 15.47 each for the return journey. Splitting fuel is often the fairest way to share the cost of a road trip, and having the number worked out in advance avoids awkward conversations at the pump.

Finding your car's real consumption

The figure in your car's brochure is measured in laboratory conditions and is usually optimistic. There are better sources:

• The trip computer. Most modern cars show average consumption on the dashboard. Reset it at the start of a long drive for an accurate highway figure.
• The fill-up method. Fill the tank completely and reset the trip meter. Next time you fill up, divide the litres you put in by the kilometres driven, then multiply by 100. Do this over two or three tanks for a reliable average.
• Owner forums and reviews. Real-world figures from other owners of the same model are usually closer to reality than the official number.

As a rough guide, small hatchbacks often use 5 to 7 L/100 km on the highway, family sedans 6 to 9, and large SUVs and pickups 10 to 15 or more.

Converting miles per gallon

If your car's consumption is given in miles per gallon (mpg), convert it first. Be careful, because US and UK gallons are different sizes:

What makes you use more fuel

Two drivers in the same car on the same road can use noticeably different amounts of fuel. These are the biggest factors:

• Speed. Air resistance rises sharply with speed. Most cars are most efficient at a steady 80 to 100 km/h, and consumption climbs quickly above that.
• Stop-start traffic. Accelerating from a standstill uses far more fuel than cruising. City driving usually costs more per kilometre than highway driving.
• Air conditioning. In hot climates the AC can add a noticeable amount, especially in slow traffic.
• Weight and roof boxes. A fully loaded car or a roof box adds weight and drag.
• Tyre pressure. Under-inflated tyres increase rolling resistance. Check them before a long trip; it's also safer.
• Idling. Waiting with the engine running burns fuel while you travel zero kilometres.

For a realistic budget, add 10 to 15 percent to your estimate, particularly if you expect traffic or will be driving fast on open highways.

Don't forget the other costs

Fuel is usually the biggest cost of a drive, but not the only one. Toll roads, bridges, tunnels and parking can add a surprising amount, especially in cities. Check the toll operator's website for current rates before you leave. In the Plan Your Route planner you can add a fixed amount for tolls and parking under Cost & time settings, and it will be included in your trip total.

Electric and hybrid cars

The same idea works for electric cars, with different units. Replace litres with kilowatt-hours (kWh):

Charging at home is usually much cheaper than fast chargers on the highway, so the cost of a trip depends a lot on where you plug in. For plug-in hybrids, estimate the electric range first and use the petrol formula for the remaining distance.

Quick checklist

• Get the route distance from a route planner (and double it for a return trip).
• Use your car's real-world consumption, not the brochure figure.
• Check the current local fuel price.
• Add 10 to 15 percent for traffic, speed and air conditioning.
• Add tolls and parking.