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Originally Posted by ps2fixer
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Looking at that truck, it's an aero nightmare. Nearly everything is a sharp edge. Sharp edges have the effect of keeping the air going whatever direction it just was, so for the bumper/air dam and windscreen, that means the air keeps going up, down, and sideways long after the vehicle dimensions have ended. The hole you are punching through the air could be 20-30% larger than it needs to be. It's also going to be a lot of work to fix, and not particularly pretty
Looking at a profile pic of a similar truck, if you were to install a full length flat undertray at the height of the transmission crossmember, side skirts to meet it, make your front air dam with a rounded bottom and sides (about a 6" radius would be a good starting point to try), and taper the undertray up to meet the rear bumper you could largely eliminate underbody drag.
Because the entire exhaust system is above this point height, though, you would need to have cutouts underneath the exhaust to ensure the exhaust didn't overheat anything.
Don't entirely block the bottom of the engine bay or you will not get very far up the road, but keeping the undertray uninterrupted until, say, the back half of the engine can increase cooling airflow by making the airflow where it does join a much cleaner and higher speed thing. You can tune this airflow exit the same way you can tune a grill block.
Ground clearance, departure and approach angles would remain unchanged, but break angle would be reduced slightly and the vehicle would appear visually a lot lower.
Installing a low profile roof box with a curved front would also help a lot. Apply the same 6" radius to the top of the windscreen and see how high it ends up, it will likely be a bit taller than your current lights, but not by much. If you make the front section clear you could mount your lights in it. Increasing the frontal area might seem counterintuitive but it's better than a square edge.
As for the side-front edges, well, you're pretty much screwed on that front.
The wheel gaps, you could fill in with mudguard skirt brushes like they do on semi's, to allow for cleaner airflow and less road spray with full articulation (but they wear out). That's at least a "real truck" thing too.
Also being a diesel, engine intake and exhaust flow is especially important as they are full airflow all the time (varying only with boost). Intake and exhaust flow improvements will result in fuel economy improvements even without fueling changes because at full load it will be running leaner. Even if boost somehow corrects itself load on the turbo will be reduced and you will get slightly better throttle response.
Is it intercooled? If not, do that. 100% do that if you modify nothing else on the engine for this truck.
Check the condition of the injectors and pump (somehow). Spray pattern is much more critical on a diesel than a petrol and you might pick up some (lost) power and reduce smoke emissions.
Google says your downpipe is the best $$$ per performance increase you can do on those trucks, but it requires firewall massaging (probably with a really big hammer)
Also, lose the stone deflector. Replace it with a piece of clear or red vinyl if you're worried about chips on the leading edge of the bonnet.