Is it worth driving further for cheaper fuel?
Chasing a cheaper price can cost more than it saves. The break-even point is easy to calculate, and once you know it, most detours answer themselves.
The calculation
Driving somewhere burns fuel, and you have to buy that fuel too. So the real question is not "is this forecourt cheaper" but "is it cheaper once the drive is paid for".
An out-and-back detour of d miles in a car doing m miles per gallon burns 2d ÷ m gallons, which is 2d ÷ m × 4.54609 litres. That fuel is bought at pump prices, and it is spread across the tankful you came to buy.
In a 45 mpg car with a 55-litre tank, every extra mile adds about 0.37% to the effective price of the fuel — roughly 0.6p a litre at current prices. Which gives a simple rule: a detour needs to save about 0.6p a litre for every extra mile, just to break even.
What that means in practice
A forecourt 2 miles out of your way needs to be about 1.2p cheaper to be worth it. One 5 miles away needs about 3p. One 10 miles away needs 6p — a gap that exists, but not often, and rarely in a direction you were already going.
And that is only the fuel. It ignores your time, the wear on the car, and the fact that the price you drove to see may have changed by the time you arrive.
Where the real savings are
The money is not in long detours. It is in choosing well among the forecourts you already pass.
The spread between the cheapest and dearest forecourt within a single town is routinely 10p a litre or more — a genuinely large sum on a full tank, available for no detour at all. Most drivers fill up at the same site out of habit without ever checking whether the one a street away is cheaper.
The second real saving is avoiding motorway services when you have any alternative. They sit far above every high-street brand, and the difference on one tank is usually larger than anything a detour will win you.
A caveat about running low
All of this assumes you have enough fuel to choose. Driving on fumes to reach a cheaper forecourt is a bad trade at any price gap — running out is expensive, and the anxiety is worth more than 3p a litre.
The sensible version is to know which nearby sites are consistently cheap and fill up there when convenient, rather than making each fill-up a decision.
Common questions
How far is it worth driving for cheaper petrol?
As a rule of thumb, a detour needs to save roughly 0.6p a litre for every extra mile driven, in a typical 45 mpg car with a 55-litre tank. Two miles needs about 1.2p, five miles about 3p.
Does the size of my tank change the answer?
Yes. The detour cost is fixed but the saving scales with how much you buy, so a larger tank makes a detour more worthwhile. Filling 55 litres justifies roughly twice the detour that filling 25 litres does.
Is a more efficient car better or worse for detours?
Better. A 60 mpg car burns less getting there, so its break-even distance is longer than a 30 mpg car, which pays roughly twice as much for the same detour.