Pickup airflow
 

I'm still not set up for CFD yet. Meanwhile, I've given considerable thought to the distribution of airflow around my truck. I finally did some napkin calculations. Based on size and shape of respective surfaces and areas I suspect to a first order approximation the air displaced by my 02 Tundra 4WD is roughly as follows.

Under: 10%
Sides: 30% each (x2)
Over: 30%

Of the 30% on each side I suspect about 10% goes past the windows and 20% goes past the lower sides up to tailgate height. When I consider gradients from higher to lower where those areas meet along upper and lower surfaces, those numbers seem consistent with how mist and rain flows over those transitions as well as points from which wake additions seems to originate at least from what I can see in mirrors. These observations give me some confidence that I may not be too far off with this approximation.

For the sake or argument, let's suppose these numbers are close. Then it provides an interesting viewpoint for where to mod the truck. An areocap could improve half the air flow (30+10x2=50%). Of that top shape seems clearly more important than side shape. This agrees with aerocap designs and performance reports from members here BTW. These numbers also suggests that wheel skirts and belly pan mods might address about 30% (10+20/2x2) of total airflow so less potential gains there.

I'm very curious what methods others might have used to break down, either approximately or precisely, the distribution of airflow over their trucks. We can't do too much comparison across makes and models of course but I think it would be interesting to compare methods nonetheless.

Cheers
KB

Your numbers look good, however the underside "suface" is the worst and therefore would represent an even larger portion of the drag. The sides are smooth and flat with few large changes in cross section. The top side of the average pickup also dispreportional with respect to the % affect on the total drag of the air passing over it. The underside will also be affected by "ground affect" in what manner I don't know? When I look at the under side of my Aerostar van I see the worst aerodynamic surface emaginable, with big things sticking out all over the place. The underside of pickup trucks is the same. So even if only 10% of the air goes under the truck it still may represent a disproportional affect on total drag... thus being a better place to look for gains? and has the added advantage of being unseen...having no affect on apearance of the truck.

One thing you need to consider is the angle rake of yer truck as referenced to the ground. ;)

Obviously this changes as you add things into the bed, gain/lose passengers, or as yer fuel tank(s) git empty.

Also...bedlength, cab size, nose size will make a difference compared to other pickups. :D

airflow
 

The subject is an important one and your numbers resonate with something I ran across long ago.
But with so many different pickups out there( and cars ) I kinda just let go of trying to get a handle on it and focus on empirical data for mods which demonstrated hard improvements.
The half-Zeppelin is the benchmark for efficiency with the upper portion and under-belly of the HONDA Dream-2 solar car,the model for everything down under.
Anything which pushes a vehicles structure in those directions is sure to benefit at the pump or charging station.

Maybe consider closing off the body lower edge to framerails with covers FF to RR; as well as lowered air dam/enclosure to push air off of steering gear, etc at front? FF/RR rake is a big deal on one of these, as well.