How to Work Out Road Trip Fuel Costs Before You Go

Last summer a friend budgeted 80 pounds of fuel for a 600 mile trip to the Highlands and back. He spent just over 140. Nothing went wrong with the car. He simply used the mpg figure from the brochure, forgot the roof box, and never counted the detours to the coast that turned out to be the best part of the holiday.

Working out road trip fuel costs is simple arithmetic, but the inputs most people use are wrong. Here is the method I use before every long drive, with worked examples in both UK and US gallons, and the handful of factors that quietly add 20 to 30 percent to the bill.

The Basic Road Trip Fuel Cost Formula

Three numbers do all the work: total distance, your real fuel economy and the price of fuel.

Distance divided by mpg gives the gallons you need.
Gallons multiplied by the price per gallon gives the cost.

Where fuel is sold by the litre, convert gallons to litres first. A UK gallon is 4.546 litres. A US gallon is 3.785 litres. Mixing the two is the most common error in online calculators, and it changes the answer by about 20 percent.

Two Worked Examples

UK example

A 600 mile round trip in a hatchback that really achieves 42 mpg:

600 divided by 42 is 14.3 UK gallons.
14.3 multiplied by 4.546 is 65 litres.
At 1.45 pounds per litre, that is about 94 pounds.

US example

The same 600 miles in an SUV that really achieves 26 mpg in US gallons:

600 divided by 26 is 23.1 gallons.
At 3.40 dollars per gallon, that is about 78 dollars.

Those are your base figures. Now make them honest.

Use Your Real Mpg, Not the Brochure Figure

Official economy figures come from controlled lab tests. On a loaded motorway run most cars fall 10 to 20 percent short of them, sometimes more.

The best source is your own car. Fill the tank to the first click, reset the trip meter, drive normally for a week, then fill again. Miles driven divided by gallons added is your true mpg. Better still, do this on a long motorway drive, because that is what the trip will be. The trip computer is a decent second choice, though many read a few percent optimistic.

The Hidden Factors That Raise Fuel Use

These are the extras that turned my friend's 80 pounds into 140. Add them to your base figure as rough percentages.

Factor	Typical extra fuel
Roof box or roof bars with bikes	10 to 25 percent
Cruising at 80 mph instead of 70	10 to 15 percent
Full load of passengers and luggage	3 to 8 percent
Air conditioning on hot days	3 to 10 percent
Towing a small caravan or trailer	25 to 40 percent
Hilly or mountain roads	5 to 15 percent

Tyre pressure belongs on this list too. Under inflated tyres increase rolling resistance, so set pressures for a full load before you leave. If your warning light keeps returning after you fill them, read why the tire pressure light keeps coming back and sort it out first. Our checklist for the week before a long road trip covers the other items worth doing at the same time.

Do Not Plan Fuel Stops Around the Cheapest Station

A lot of budgeting advice says to map every fuel stop to the cheapest station on the route. I think that usually costs more than it saves.

Take a 15 gallon fill. A station 5 pence per litre cheaper saves about 3.40 pounds on a UK gallon fill of that size. A 6 mile detour to reach it, there and back, burns close to a pound of fuel and adds 15 minutes, so the net gain is about two pounds. On a long day that also means less rest and more tired driving, which matters far more than the price on the sign.

What does save money is avoiding motorway service stations where prices are often noticeably higher than in nearby towns. Filling up in a town just off the junction, when you are stopping anyway, gets most of the saving with none of the detour. Schedule fuel stops to line up with driver changes; our guide on splitting driving on a long road trip suggests swapping roughly every two hours.

Add the Miles You Have Not Planned Yet

Route planners measure the line from A to B. Real trips include the hunt for the hotel car park, the drive to the restaurant, the scenic detour and the wrong turn in a city centre.

My rule is to add 10 percent to the planned distance for a simple point to point trip, and 20 to 25 percent for a touring holiday where you are based somewhere and exploring daily. On a 600 mile plan that is 60 to 150 extra miles, which is real money.

Build the Final Budget

Put it together in four steps:

Planned miles plus your extra mileage allowance.
Divide by your real mpg.
Add the percentages from the hidden factors that apply.
Multiply by the fuel price, using a slightly higher figure than today's local price.

For the UK hatchback above, 600 miles plus 20 percent is 720. At 42 mpg that is 17.1 gallons, or 78 litres. Add 15 percent for a roof box and a full car and it is 90 litres. At 1.50 pounds per litre, the honest budget is about 135 pounds, very close to what my friend actually paid.

Frequently Asked Questions

How do I convert mpg to litres per 100 km?

Divide 282.5 by UK mpg, or 235.2 by US mpg. A car doing 42 UK mpg uses about 6.7 litres per 100 km.

Is it cheaper to drive or take the train for a road trip?

For two or more people, driving is often cheaper on fuel alone. Add tolls, parking and wear on the car, and compare against advance rail fares before deciding.

Should I fill up before a long trip?

Yes, in a normal town station the day before. It avoids paying motorway prices on your first stop and means you set off without an early detour.

The Practical Takeaway

Measure your own mpg, add 10 to 25 percent to the route distance, and add the percentages for roof boxes, speed and load before you multiply by the fuel price. Fill up in towns rather than at motorway services, and stop chasing a few pence at the cost of detours. You will end up with a fuel budget that matches the receipt, not the brochure.