Yesterday I had to drive to a nearby town, just 20 minutes away, and noticed that every single petrol station there was a good 5p per litre cheaper than my town. I might plug this into a map.
as much as I try to stay away from Google products: waze shows this on the usual driving map quite well
The variation in even a couple of miles can be pretty big. I almost never go out of my way to visit a cheaper petrol station because that's usually a false economy, but there are definitely some local places that I favour or avoid because they're almost always cheaper/more expensive than the surrounding ones.