In addition to listing gas pumps in your area with low prices, Gasbuddy also has a national map of average gas prices by county which is titled USA National Gas Temperature Map
The use of the word "temperature" is not accurate but is probably effective in communicating what the map is about. It's a dynamic map and you can zoom to your specific region. Southern Californians would have to drive a long distance to find cheaper gas.
Abhijeet Chavan is co-founder and co-editor-in-chief of Planetizen.