Figured I would give it a run before I supercharge it. I've been running long distance for work the last couple of weeks in Louisiana so I've been on really flat ground. Only down side is I've been running 75-80 mph. First I tested 93 octane. I averaged around 20mpg. Had a best of 22.5 for a while. Non of this is with cruise set, this is with me decelerating up hills and accelerating down hill to compensate for mph loss. So pretty much nest case scenario. I ran 3 tanks out with pretty much exact results each time. Then I tested e85. (2 tanks back to back). With it I averaged around 15.5 with the exact same driving habits. Still not horrible for the speed. And a little back ground on truck. I am leveled with 295/55-20 bfg km2's. Also have a full size tool box with roughly 350lb of tools. No point in the post, just figured I would share my results. In a week or two it will never happen again. Haha.

 

With a stock tune mine gets better mileage on 87 then 93. Actually it doesn't seem to like premium.

 

I started running 93 a while back after pulling my camper. I notices when under hard acceleration I could hear detonation. I haven't noticed any mpg from 87 to 93.

 

This is in my 14' CCSB 6.2 All Terrain with 93 octane:

On the highway, empty truck, at an average of 70mph I get 22mpg hand calculated, around town it seems to get about 15mpg regardless of how I drive.

On the highway, open trailer with my track car on it, bed full of gear (~500lbs in bed, and trailer/car weighing ~5500lbs total) I averaged 15mpg at about 70mph.

 

That's the best I've heard from a 6.2 truck. Hell gm doesn't even rate them at that. And we all know how optimistic manufacturer ratings are. Both if my bosses have 6.2 trucks. One is a 2wd denalli the other is a 4wd high country and they both have tried to get the best mpg they can and the one with the denalli has only been able to get 19 on the highway. We all do the same work in the same area. The high country has a best of 17. But he has mud terrains and leveled like I am.

 

My tires are all seasons - and a hair smaller than stock but other than that, the truck is bone stock with 30k on the clock. Perhaps they pick up a bit of mileage after they start racking up some miles.

I dont do a ton of highway driving without the car behind me, but my base for the 15mpg with the trailer was a ~1600 mile (round trip) haul down to VIR from New Hampshire, and another ~900 mile trip to NJMP from NJ.