What kind of gas mileage will a 1999 gmc suburban 2500 7.4 liter v8 get?

Expect very poor gas mileage from a 1999 GMC Suburban 2500 with the 7.4L V8. You're likely looking at 8-12 MPG in combined city and highway driving. Highway driving might push it closer to 12 MPG, but city driving will likely be significantly lower, possibly even under 8 MPG. This is heavily dependent on driving style, terrain, and vehicle condition. Anything significantly better than 12 MPG would be considered unusually good.