I'm glad you asked this question.
I don't have a Beetle myself, but I wouldn't say no to one. Kids' tuition, house payments - you get the idea. No time or money just yet.
But: Hot VWs magazine picked up the idea of
building an engine, optimized for fuel economy. The project dropped off the radar a couple of years ago when fuel prices dropped, but prices are heading back up - maybe they'll get back into it, hey?
Anyway. Their stocker racked up a dismal 22mpg as found. With a chassis tune and a fresh alignment, they got up to about 27-29mpg. Big improvement!
Then came the engine. Contrary to what you might think, the motor they built is no slug. It makes good torque, it gets a big power bump. A little more stroke makes for good twist. Their assessment was that the car with the new motor was downright fun to drive.
Hot VWs' project was almost exclusively engine mods, and a "Freeway Flyer" tranny swap. They added a small spoiler to aid airflow separation, which was good for 2mpg, this after having already gotten solidly into the 30s. But bellypans, rear wheel skirts, smoothie wheel discs? None of that was ever done.
At its best combination, with a CBPerformance throttle body injector, the Freeway Flyer and the spoiler, the Beetle would tool down the highway at 65mph, delivering 36mpg. That's pretty good by any measure.
Do all the aero mods you can think of, and I bet you could get it into the 40s and keep it there.
CBPerformance has the entire run of articles archived on their site. I used to have them all in the mag, but needed to make room for more comic books.