How to calculate the gas cost of a road trip

01

The gas cost formula, with an example

Gas cost = distance ÷ mpg × price per gallon. The distance is what your map app shows; the mpg is how far your car goes on one gallon; the price is what you pay at the pump. For a round trip, double the distance.

Our example: a weekend trip of 240 miles each way, a car that does 32 mpg on the highway and gas at $3.60 a gallon. One way needs 7.5 gallons, the round trip 15 gallons, so the fuel costs $54. With four people sharing, that is $13.50 each. These are example values to show the calculation, not current prices.

02

Find your car’s real mpg

The window-sticker mpg comes from standard tests. Everyday driving – short trips, cold mornings, highway speed, a loaded car – usually uses more. Your own number makes every trip estimate more accurate, and it takes only two fill-ups to get it.

1. Fill the tank completely

   Fill up until the pump clicks off and note the odometer reading.

2. Drive as usual

   Use the car normally until the next fill-up; partial fill-ups are fine in between.

3. Fill up completely again

   Note the gallons you put in and the odometer. Miles driven since the first full tank, divided by all the gallons added since, is your real mpg.

4. Let the diary do it

   In Fuel Studio, open “Fill-ups”, enter each fill-up and tick “Filled up to full” for full tanks. From the second full tank on, your average mpg and the cost per 100 miles appear at the top.

03

How to split gas money fairly

The simplest fair rule for a shared trip: divide the fuel and tolls by the number of people in the car, the driver included. Enter everyone in “People sharing” and the per-person amount appears under the total.

• Someone joins for part of the way: work out their distance separately and add that share to them.
• The driver brings the car: many groups let passengers cover fuel and tolls while the driver covers wear and tear.
• Carpooling to work: calculate one day, then multiply by the days you share – it is easier to settle monthly.
• Rental car: add the rental fee in “Tolls and parking” if everyone shares it too.

04

Tolls, parking and the costs people forget

Fuel is usually the biggest variable cost of a trip, but not the only one. Toll roads, bridges, city parking and ferries can add up quickly. Put them in “Tolls and parking”: Fuel Studio adds them to the total and includes them in the split. Use the toll calculator of the road operator for current rates rather than an old figure.

Electric vehicles and hybrids change the picture: their energy cost per mile is often lower, but public fast charging can cost much more than charging at home. The “EV vs gas” tab compares both for your driving.

05

Common mistakes when estimating gas cost

Most wrong estimates come from three numbers. The first is the sticker mpg: it is measured on a test cycle, and real highway driving at speed, with luggage and the A/C on, often uses noticeably more. Use the figure from your own fill-ups whenever you have it.

The second is the distance. Map apps show the shortest route by default; the detour to a rest stop, a city loop to find parking or the drive from the hotel to the beach are not in it. Adding ten percent to long trips is a sensible safety margin.

The third is mixing units. A UK mpg is about a fifth larger than a US mpg because the imperial gallon is bigger, and European specs give litres per 100 km, where a lower number is better. Pick the right unit next to “Fuel economy” instead of converting by hand.

06

Make the trip cheaper

• Drive steadily and a little slower on the highway – fuel use rises noticeably at higher speeds.
• Take off roof boxes and bike racks you do not need on the way.
• Check tyre pressure against the sticker in the door frame before you leave.
• Abroad in Germany, France, Spain or Austria, the “Prices” tab shows live pump prices nearby, so you can fill up where it is cheapest instead of at the first motorway station.