As part of building my own truck-top camper, I got into researching aerodynamics of vehicles in order to try to reduce loss of fuel efficiency. The most interesting ideas I found were that aerodynamics don't matter much on most vehicles until they pick up significant speed.

Most automobiles are pretty heavy, so the engine has to do significant work just to get it to move. At a certain point, the vehicle can change gears to get the engine to do less work and use less fuel. But around the same time, the force of the air is increasing. By the time an automobile goes over about 50mph, the air forces are getting increasingly strong, and the engine has to work harder to keep the vehicle moving. At this point, beginning to lower the air's coefficient of drag on the vehicle will lessen the work the engine needs to do to keep the car moving at speed. So you can optimize the design of the vehicle's exterior to reduce the drag coefficient, which will reduce things like flow separation and turbulence, creating fewer rear pressure zones and causing less drag.

So you might wonder, why aren't more cars teardrop-shaped like the airfoil? The answer is, it depends. Most people want something that looks good more than they want efficient operation at speed. But sometimes having more drag actually helps. For example, the Lotus Elise: while it is smaller and looks more sleek than a Tesla Model 3, it actually has a much worse drag coefficient than a Tesla Model 3. The Lotus has way more force acting against it at speed than the Tesla. But the Lotus is a sports car, and sports cars benefit greatly from increased traction, and you can get more of that traction by increasing the downforce on the car. So the Lotus's design sacrifices top-speed drag coefficiency in order to gain some downforce which helps traction when cornering at speed.

What about pickup trucks? Even though modern pickups actually have lots of subtle design changes to improve drag coefficient, they all tend to have open beds, which is terrible for drag. It creates this giant messy turbulent pressure area in the bed which drags on the tailgate and the rest of the car. By adding a truck topper, the drag is significantly reduced, but you don't see most trucks driving around with a topper on. But trucks naturally have worse gas mileage, so nobody really thinks twice about the aerodynamics.

(To be fair, the air's impact on gas mileage is minimal unless you're going quite fast. But for trucks with extremely bad gas mileage, like 18-wheelers, it makes much more difference. That's why they often have airfoils on the front of the truck, gaps between cab and container closed, and skirts to reduce drag from the undercarriage. Strangely though, the biggest improvement to reduce drag coefficient actually comes from modern European big-rigs whose containers are actually tapered like a teardrop. The rear of the vehicle's shape makes the most difference to how severe flow separation is, and thus how big of a pressure area develops, pulling on the rear of the vehicle. If we wanted to make trucking more fuel efficient globally we'd change the shape of the containers to be more like teardrops, but that would make handling and shipping them much more awkward)

You'll usually only see these effects on automobiles at higher speeds, due to the vehicle needing to overcome gravity before the air forces build up. Lighter vehicles (say, bicycles) with less impact on them from gravity will be impacted earlier (at lower speeds) by the force of the air, so optimizing drag coefficient is much more important, which is why bicycle racers have to put so much into aerodynamics at significantly lower speeds than an automobile. Interestingly, the drag coefficient on a bicycle and rider is actually equivalent to that of a small car.