Originally Posted by 360ci
There should be a sticker of sorts in/around the glovebox (if equipped) that states axle ratio. As you have the 450, I'll assume at a minimum either 4.10 or 4.30 gears are installed.
As for economy I can't see you doing much better than 15mpg highway (65mph) empty. 100% city driving I'd put it around 10-12mpg depending on the amount of lights, turns, etc. At least the 5 speed manual is more efficient overall than the automatic. Run synthetic fluid in the differential as it might lower friction enough to warrant a slight mileage improvement.
Only compare the duramax with similar tire size, axle ratio and GVWR. Comparing your 450 to a 3500 GM DMax isn't the way to do it.
12/15 sounds more like it. Thanks.