I just drove from SE Texas to SE Minnesota. Got back this afternoon. Gas prices in Texas were about $4.20 a gallon. Texas was the most expensive state, but not a lot higher than the others. In Arkansas I filled up for $4.05 a gallon. And in Missouri and Iowa I saw gas occasionally at $3.99 or even $3.89, but only when I had a mostly-full tank already. I think I paid $4.09 in Missouri. It's higher in Minnesota but I haven't seen that many stations so don't have a feel for it; maybe $4.20? gasbuddy.com says it's $4.09 at Costco and Fleet Farm, and $4.24 at most other local stations in Rochester.
Most of the Texas stations had separate prices for cash and credit. Usually 10 cents difference, but sometimes 20.