I have known some people who because fuel was a penny cheaper at a supermarket station they made a four mile detour to take advantage of it.
Does it stand up to logic, This was in the days when fuel was about 70p per litre (1998). Their car did around 30 to the gallon (6.6miles per litre) so cost fuel cost per mile was about 2.3p per mile.
So four miles of fuel would cost them 9.2 pence. That would mean they need to fill up with a minimum of 9.2 Litres (which is about 2 Gallons) to break even.
If you look at whole cost motoring (time, tax, insurance, wear and tear etc) the AA reckon it was costing on average about 37p per mile based on 10K miles annual usage.
So to cover the real running costs of going just 4 miles to save a penny per litre would actually cost you £1.48!
At today's prices it's about 60p per mile. So is it really worth trying to look for penny savings on fuel? For most of us any detour would have to be less than a mile and for at least 60 Litres.
Obviously the benefit improves if the price difference is more, or the car is a thirsty model.