We use a phone app called Gas Buddy. It lets you see the gas or diesel prices in the area you are in or you can enter another city so you can decide to wait for a better price further along. It is user updated, so you can enter prices you see at stations. There is some kind of points reward for updating prices, but I haven’t really got into that part. We are able to find the cheapest fuel using this app.
One thing we are finding here in BC, Canada…if there is a Costco gas bar (only gas, unfortunately) in the area, the prices are way lower at all the stations there. If you have a membership for Costco, it will usually run about 1 cent cheaper for you there. But, if not you can still get it at one of the other gas stations in the area for a pretty good price.