That still means absolutely nothing. It's impossible to know your MILES per GALLON when you're not reporting the number of MILES you are driving for every number of GALLONS of fuel you are using. MPG can not be calculated by saying "I can go from my house to Uncle Joe's house, then to Walmart, then to Mama's house and then back home on 3/4 of a tank of gas so I guess that's (good/ok/poor) mileage". Miles per gallon is simply that- MILES per GALLON. Your Tahoe may or may not be getting the fuel economy that it should be, but we can't help until we know actual numbers. Fill up your tank and reset the trip odometer. When you fill up again, write down how many miles your trip odometer reads as well as how many gallons it took to fill it back up. Post those numbers here. You can also reset the fuel economy gauge on the dash when you fill up and you should get about the same figures.
As far as using E85, I'm betting it would get worse MPG. But, I've never ran E85, just the E10 that's pretty much everywhere in my area. But I try to avoid even that and use 100% gasoline where available.
