I'm thinking of looking into a 6.9 and wondering what kind of mileage they get or with an aftermarket turbo kit.
I've got the 4.10 gears and NA engine, it does ok but not as good as it could be doing. Only mid teens for mileage, then again I am still stripping my brother's jury-rigged handiwork off of it.
The 90 7.3 IDI I had was getting 11 mpg after I replaced the injector pump for the highest horse power pump for that engine and it smoked at higher elevation but had more power than the high altutude injector pump. It had a Z5 transmission 4X4 and 4:10 gears.