Should I fill up at SHELL, Bp or Speedway?

I just got my check and one of the first things I do is fill up my tank. I have a 1997 Cavalier and I get 24 mpg City. Should I fill up at BP, Shell, or Speedway? I usually go to BP but maybe I'll get better milage/last longer somewhere else? Those are the only stations I trust, others are Sheetz and Getgo.