Petrol fuel cost per trip:
formula and worked example

Budgeting fuel before you set off

Fuel is the largest variable expense on a road trip, and it is the one you can estimate in advance with reasonable accuracy. The estimate turns on three numbers you can find in minutes: the distance, how far your car goes on a litre, and the current pump price.

The result is a budget, not a promise. Traffic, hills, a loaded boot and a running air-conditioner all push consumption up, so the calculation is best treated as a baseline from which you add a margin. Still, having the baseline helps you split costs with fellow travellers, compare routes, or decide whether the train is cheaper.

It also helps to think in two layers. The first is the pure fuel bill, which this formula gives you. The second is everything else on the road: tolls, parking, food and wear. Keeping them apart stops you from underestimating a trip because the petrol figure looked small.

The formula and what to enter

• Distance: the route length in kilometres, doubled if you are budgeting a return trip.
• Km per litre: your car's real-world economy, ideally from your own tank-to-tank records.
• Fuel price: the price per litre you expect to pay, which may differ along the route.
• Result: cost in your currency. Intermediate result: litres consumed.

Notice that the formula separates into litres first, then cost. Keeping the litres figure visible lets you check it against the tank size and plan refuelling stops.

If you only know your economy in litres per 100 km, as many manuals state, invert it: 100 divided by 6.7 litres per 100 km is about 15 km per litre. Entering the wrong form is a classic cause of a result that is far too high or far too low.

When prices differ along the route, split the trip into legs. Work out the litres for each leg, price each at the pump you expect to use, and add the results. It takes a few more lines but removes a source of guesswork.

Worked example: 430 km at 15 km/l and 105 per litre

A family plans a 430 km one-way drive in a car that averages 15 km per litre, with petrol at 105 per litre.

1. 1Litres required: 430 ÷ 15 = 28.67 litres.
2. 2Cost: 28.67 × 105 = 3,010.
3. 3Cost per kilometre: 105 ÷ 15 = 7.
4. 4Check: 430 km × 7 per km = 3,010.

With four people in the car, the fuel share is about 752.50 each one way. On a return journey the total doubles to 6,020, which is the number to compare against other transport options.

The 28.67 litres also tells you something about the tank. In a car with a 40-litre tank that is about 70% of a full tank, so one stop is probably enough on a one-way trip, while the return needs a second fill-up somewhere.

Interpreting the result honestly

The calculation assumes the car achieves its average economy over the entire route. A figure measured mostly in city traffic will understate highway driving at moderate speed and overstate it at high speed, so the same car can score quite differently on different trips.

• Steady cruising on open roads tends to be the most economical pattern for a petrol car.
• Stop-start traffic, steep climbs and a fully loaded car all reduce km per litre.
• Running the air-conditioner raises consumption, often more in slow traffic than at speed.

If you have no record of your own, take the brochure figure and trim it by a tenth or so to be on the safe side.

One more point: the figure is your marginal fuel cost, not the true cost of driving. Depreciation, insurance and servicing all depend on kilometres too. If you are comparing a drive against a train or a bus, add a per-kilometre allowance for those, or you will make the car look cheaper than it is.

How distance and economy move the bill

A 10% longer trip costs exactly 10% more: 3,010 becomes 3,311. Economy is more punishing because it sits in the denominator. If the car manages only 13 km per litre instead of 15, the same trip needs about 33.1 litres and costs roughly 3,473, an increase of around 15% from a drop of just two kilometres per litre.

Mistakes in trip fuel budgets

• Using the brochure economy for a loaded car on a hilly route.
• Forgetting the return leg when the trip is a round trip.
• Assuming one fuel price for the whole journey, when prices differ between states or on highways.
• Ignoring idling and detours, which add kilometres you did not count.
• Mixing miles per gallon with kilometres per litre.

Turning the number into a plan

A practical approach is to calculate the base cost, add a 10% margin for traffic and detours, and then decide on a fuel stop. A 430 km trip needing about 29 litres suggests one refill is enough for most tanks, but leaving the starting tank at least half full avoids a forced stop at an expensive pump.

• Find the real km per litre from your own logs.
• Add 10% for traffic and detours.
• Price the fuel at the highest pump you may use.
• Double the figure if the trip is a round trip.
• Add tolls and parking separately.