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A journey is rarely one kind of driving, so this one is planned in legs. Each leg can carry its own consumption figure, and the total is worked out by adding the fuel and adding the distance rather than by averaging the legs, which does not work on figures like these.
A name and how far it is: Motorway to the coast, 184. Add a third figure to give that leg its own consumption in the unit chosen below: Town traffic, 12, 22. Legs without one use the figure for the whole trip.
There and back doubles every leg.
Your own measured figure, not the brochure one.
Only a label. Nothing here is converted between currencies.
6.28 US gal · 21.66 USD
242 mi in 3 legs
Averaging the leg figures would give 7.74 L/100km, and the journey actually returns 6.1. Consumption figures are rates, and a rate has to be weighted by the distance it applies to before it can be combined.
Fuel only. Tolls, ferries, parking and the coffee at the services are not in these figures, and the price you type is the one used for the whole route — fill up across a border or on a motorway service area and the real bill moves with it.
A long drive is several different drives wearing one name. Two hours of steady motorway, an hour of hills on a B road, half an hour of stop-start through a city on the way in — those stretches do not consume alike, and a car that returns 45 mpg on the first can be down in the twenties on the last. Entering the route as legs lets each stretch carry the figure it deserves, and leaves you with a table you can read rather than a single total you have to trust.
The leg view answers a second question the total cannot: whether any individual stretch is longer than the fuel you will be carrying. Knowing that the run across the moor needs eleven gallons is what tells you to fill up before it rather than halfway across.
Take 184 miles of motorway, then 46 miles of coast road at a measured 34 mpg, then 12 miles of town traffic at 22 mpg, with the car returning 42 mpg the rest of the time. The motorway leg needs 4.38 US gallons, the coast road 1.35, the town crawl 0.55: 6.28 gallons for 242 miles.
Set the journey to there and back and every leg doubles, along with the fuel and the bill. What does not double is the consumption figure — 12.56 gallons over 484 miles is still 38.5 mpg, because doubling a journey cannot change what the car does per mile.
The total above is 38.5 mpg, and the three leg figures are 42, 34 and 22. Their plain average is 32.7, which is a worse number than the journey actually achieved, because the thirsty legs are short and the plain average pretends all three matter equally. Weighting matters more here than it looks: the twelve-mile crawl is five per cent of the distance and gets five per cent of the say.
So the planner never averages the legs. It converts every leg to fuel, adds the fuel, adds the distance, and derives one consumption figure from the two totals. When the leg figures differ enough for the plain average to mislead, the tool prints both and names the gap, because seeing the two side by side is the fastest way to stop trusting the wrong one.
One price is applied to the whole route, which is a simplification you should undo yourself when it matters. Motorway service areas charge a premium over supermarkets, crossing a border can move the price by a third, and a long trip crosses several pricing regions. If your route does that, split it and price each part at what you expect to pay there.
Only fuel is counted. Tolls, bridges, tunnels, ferries, city charges, parking at the far end and anything eaten at a services are outside the figure. So is the effect of loading the car for a holiday: four people, a full boot and a roof box are worth a noticeable share of your economy, and a roof box left on when empty is worse value still. If the trip is loaded, measure the car loaded, or plan on a figure a little worse than your everyday one.
Use whatever your own record shows for similar driving, and be pessimistic where the route differs. A brochure figure will flatter you on hills, in the cold and with the car loaded, and every one of those applies to most long journeys.
Compare each leg against a usable tank, which is smaller than the rated capacity because nobody sensibly runs to empty. If a leg needs more than about three-quarters of the tank, plan a fill before it rather than looking for one during it.
This works in fuel burned, which is the honest figure for the journey. You may leave home with a full tank and come back with a quarter of one, so what you actually spend on the road depends on where you choose to fill up.
Yes, provided you feed it a consumption figure measured while towing or loaded. Trailers and high-sided vehicles pay for air resistance, and their economy falls away sharply with speed, so a figure taken at 55 mph will not describe 70.