I need help with a 92 f-350 with the 7.4l turbo diesel, the thing only gets about 13 MPG without anything attached to it, the engine is running about 2800 RPM at 70MPH.

The rear end has a tag on it that reads 106 9 and then 10 10 2 6 F 07 any idea what gearing this has and what it can be replaced with?