Is cheaper fuel worth the drive?
A forecourt 3p a litre cheaper sounds like a win. But if it's several miles away, the fuel you burn getting there can eat the saving.
The sum
There are 4.546 litres in a gallon, so a car doing M miles per gallon uses 4.546 ÷ M litres per mile.
Cost of the extra drive = extra miles (there and back) × 4.546 ÷ mpg × price per litre
Saving on the fill-up = litres you buy × difference in price per litre
If the saving is bigger than the cost of the drive, it's worth it.
An example
Your car does 45 mpg, you buy 40 litres, and fuel is about 175p a litre. A forecourt 3p cheaper is 3 miles further away, so 6 extra miles there and back.
- Fuel per mile: 4.546 ÷ 45 = 0.101 litres, about 17.7p a mile
- Cost of the extra 6 miles: about £1.06
- Saving on 40 litres at 3p: £1.20
You'd be about 14p better off: hardly worth the time. At 5 miles further (10 miles there and back), you'd lose money.
The break-even rule of thumb
For that car, every mile of extra round trip needs roughly 0.45p a litre of saving on a 40-litre fill-up just to break even. The smaller your fill-up or the thirstier your car, the bigger the saving needs to be.
Let Spotapump do it
Spotapump Plus works this out for every forecourt near you, using real road distances (not straight lines) and your own car's mpg, and shows what each one really saves compared with your nearest. Try the live map.
Questions
Is it worth driving further for cheaper petrol?
Only if the saving is bigger than the fuel used getting there. For a 45 mpg car buying 40 litres at around 175p, a forecourt 3p cheaper but 3 miles further away saves only about 14p once the drive is counted.
How much fuel does a mile cost?
Divide 4.546 by your car's mpg to get litres per mile, then multiply by the price per litre. At 45 mpg and 175p a litre, that's about 17.7p a mile.