"I'm just saying that after the gas crisis of 1979, some cars, like the Rabbit and some economy Jap cars, seemed to get better mileage than many seem capable of now. I have no idea why. I'm no engineer."

Well, there are several factors at work, affecting the mileage of current cars, and reducing compared to the late 70's early 80's economy cars you speak of.

First, those cars weighed much less than current cars. A vw rabbit of the time weighed between 1800 and 2200 lbs, depending on the year and configuration (2-door, 4-door, cabrio, ect.). A current vw rabbit runs around 3100lbs. This increase in curb weight requires more force and therefore more power to move F=M*A Force needed=Mass*Acceleration

The increase in weight is due to all of the niceties that are expected on current cars, as well as the structural reinforcements required to pass modern crash tests and deliver a sturdy chassis, and also the size of current cars.

Another factor affecting the mileage is the power of the engines in current cars. A typical family sedan will have in the range of 150-200 hp. A economy car from the late 70's to early 80's would have half that amount or less. The diesel rabbits in non-turbo form had a whopping 49hp. That's not acceptable for modern driving habits, nor the weight of modern vehicles.

Another factor is the speed that most people drive nowadays. No 55mph speedlimits anymore. Most people drive 70+ on the highway. The wind resistance at higher speed requires significantly more power to overcome.

Last factor I'll mention is the emissions controls on modern cars. When you lower NOx emissions through egr controls and stratified burns in the engine you reduce the efficiency, and therefore the mpg rating.