I bet, the Tahoe was a winner!
Nothing official, but mileage sucks.
Stock the truck had 3.42 gears, an overdrive ratio of .69 and 285's... it was pulling down 18-19 MPG no problem. That gearing combo just allowed the engine to spin at low RPM's everywhere. It had enough torque to pull it off.
Now with 4.88's and 37's I'd be surprised to see more than 12 MPG... more than likely 10-11 MPG average. I haven't driven it enough to get some actual numbers, so that's a guesstimate.
At freeway speeds (75+ MPH) truck has a hard time staying in OD on anything but flat ground, which is killing the mileage. I would like to rebuild both the engine & transmission and hope I'll gain a good deal more torque with a 383, helping the truck stay in OD.
I realize the 6.5l diesel engine is a turd from a towing/performance standpoint, but I wonder how a 6.5 equipped GM truck would do with the same build and gearing that worked with the diesel.