2004 5.3 Z71 is getting horrible MPG, no SES light, new acdelco plugs like 150 miles ago, 40 psi on stock size tires, I'm getting 10 city, 13 hwy, hand calculated driving with 2k shifts. Truck had a bullydog on it before I believe, could it be set on a 93 or 89 octane tune or something? Cats clogged?