Hey Ferm, or anyone that knows diesels

[oteps]

I would spin the rear and check the gearing. Maybe someone changed it. did u replace the down pipe when u did the exhaust? Its the one between the firewall and engine.
U may be better off buying a rusted out 4x4 truck with the gears ratio you want to use for parts. we have a lot of them up here.
I have a 99 f350 7.3. I was getting around 18 mpg. haven't checked it since stage 1 injectors and chip.