Citgo is being beaten by:

Diamond Shamrock
Chevron
Shell
Exxon

and the major grocery stores:
H-E-B
Randall's

In fact, there's only 1 gas station within this zip code where a Citgo is cheaper, and that's a Texaco that's off the beaten track, that has a mini mart, a pizza takeout, and a paintball (formerly a driving range) field attached to it.