Tail fins add weight and no volume obviously. The drag part is probably minimal.
Aerodynamics for mpgs is not that much of a thing any more really, but weight is. Cars are all pretty aerodynamic now, 0.30 is common.
Cleaning up the underside of the car is important, as well as keeping air out from underneath. And since a lot of cars can exceed 140 mph today, there is emphasis on downforce at speed which causes drag.
The Pinto and Vega weren't crappy general designs but their execution was awful. Car makers discovered you don't save much money just by making a car smaller, you still have the labor costs, all you save is on some steel and glass. Had they made those cars basically smaller versions of larger cars in effect they would have cost nearly as much.
Then they discovered that making a car smaller didn't always mean a lot better mpgs in the real world. My first wife brought a Chevy Chevette with her, a new one, and it was hard pressed to get 26 mpg highway. It was an automatic, and horrible.It had the two barrel carb on it. That was a bad car, rear wheel drive, and not much room.
The Pinto and Vega made an interesting study in contrasts.
The Pinto was like a small version of a big, standard car. Cast-iron straight-4 (rather than V-8), smaller transmission, the same layout in just about every way, but smaller. My sister had a Pinto wagon with wood-grain vinyl on the sides. "Pinto Squire," I think it was called. She loved it, and it drove pretty well. IMO, the only thing wrong with the Pinto was the unsafe mounting of the gas tank. Of course, that was a very serious flaw which Ford could have fixed rather than deciding that it was cheaper just the pay the damages in the inevitable lawsuits.
The Vega was an attempt to really make something new. Aluminum-block engine, rack-and-pinion steering, some performance aspirations. And it was a piece of crap. The aluminum-block engine, oddly, had a cast-iron head. It was a very fragile engine, overheated, sprang leaks, etc. It was later replaced with a cast-iron engine. Chevy had a rep back then of letting their customers do the development work, so that by about the 3rd (and often final) year of a body style, everything would work pretty well. That didn't work with the Vega. By the time it got sorted out, its image was fatally damaged.
The was a high-performance Vega, the Cosworth Vega, introduced in 1975, the 6th year of production. It was too little, too late.
