I have a 99 GMC with the 5.3. I got 180 miles on my tank and it is now just barely above 1/4 left. I filled it up Saturday morning. My tire pressure all around is 36psi and I am running stock265/75/16 Mastercraft A/Ts. I have a TBS and a Spectre CAI with a Hypertech30005 programmer. I'm starting to think this is an engine issue. Any ideas?