The ColoradoFans forums have an extensive compiled thread of what combinations of susp upgrades are recommended to properly accomplish various lifts. Really a rich database of success factors with components, spacers, tire sizes, fender rub, etc.
Regarding a simple level-lift and air dam delete, the fuel economy hit should be minimal. I wrote it up here for 1-1/2" suspension lift -
https://www.coloradodiesel.org/threads/2-8d-off-pavement-tech-report.65513/#post-112897
Once you're talking 3+ inches of lift and going up a few tire sizes, it's reasonable to assume loss of fuel economy. Both due to aerodynamics, and in city driving, due to the large added rotating weight of tires. I've been researching a very modest tire increase from 30" to 32" (no change in width), which is just possible with my level lift. Some tire choices only add a few pounds, while other AT/LT tires at the same size add tens of pounds at each corner. So if you have a lot of city mileage, picking the wrong tire (by weight, regardless of size) could impact more than you anticipate. That aspect will reveal whether you're a mudder at heart, or mindful of city mileage at heart
