Just for some fun tech to add here -and random stream of consciousness:
5th gen drag coefficient is 0.36. It's just a bit lower than a Tacoma at around 0.39. And much better than a Wrangler JL at .46.
Fuel efficient cars like the Prius or Tesla model 3 are more like 0.25 range.
Also to figure out the actual force required to push through the air you multiply the coefficient "Cd" by the frontal area and then by air density times velocity squared divided by two. So Cd*reference area*[(air density*velocity^2) / 2].
When you start screwing up the air flow with things like roof racks and tents, it gets a lot worse.
Smooth air flow over the surface like this probably keeps the windshield clean because the bugs are heavier than air and so they'll tend to be thrown out of the air flow as it changes direction. The angle of the windshield compared to the hood is basically why the bugs end up on the windshield and not flowing over top. It's very possible that the small bug deflectors really do make a big difference by deflecting air up before the bugs are ejected from the air flow. They probably also cost you about 0.5-1mpg at highway speeds.
GM published a pretty interesting article about this and how things like bug deflectors and fender flares can really hurt your mpg. They also said that a mesh tail gate is the worst possible solution, open tail gate is also worse than closed, and closed is best. Closed with a soft bed cover is actually the best possible truck configuration over a hard cover. And running boards actually help quite a bit if they're designed correctly.
For a 4Runner - I've always been curious about having a front air-dam to see how much it helps. Something remove able or maybe even an electric flip down part of the front skid plate. I think for the lifted 4runners like mine it could help significantly, especially if you made a similar drop down side skirt that became part of the rock slider when it lifted up. If I could net 2-3mpg on the highway, I'd probably do it. OTOH - the better solution to all of these areas is height adjustable suspension. I've tried, failed, but haven't totally given up on that. I can't work out a viable front solution that's doesn't' either involve dropping down to a small shock diameter and using a bag-over coil, or some sort of complex linkage based solution. The rear is easy.
finally, random though #? - for those of us who are older - I love seeing a windshield covered in bugs. It reminds me of how it used to be. Before we killed all the insects. A clean windshield is kind of a scary thought - because it means that there's no insects around. And I'm sure many of us have noticed the significant decline over the last 20 years in insects on our cars. I don't think it's all the result of better air flow. And I'm troubled that it's a sign of how we're killing our ecosystems in ways that we don't really measure very well.
