Driving somewhere to fill the tank 5p/l cheaper probably waste of time but unless all your trips are very short you usually can find a petrol station along a commute roue or along the route of a long leisure trip which is cheaper than one closest to your home. When you see prices in your navigator it's much easier to do.

Agreed. Most of the time the cost of driving out of your way exceeds the saving you would make. However, there's a fuel station near me that is 19.8% cheaper than one only 1.6 miles away from it, and that's a thing that is worth knowing.

https://xkcd.com/951/