Hi,
I've always been an aircooled fan, though I've never owned a Beetle. Time and money, I guess, though the initial buy-in on a VW is, I know, pretty low.
Anyway. Part of my enthusiasm has been expended reading "Hot VWs" magazine, and a couple of years ago they ran a series of articles on what they were calling a "mileage motor." The goal was to achieve 40mpg while keeping up with California traffic - not poking along at 45, but hurtling with everybody else at 70.
They never made it. But they got darned close.
Link to the
entire series of articles, as hosted by CB Performance parts. Interestingly, to optimize the engine for economy, they wound up building one that was also impressively more powerful, WAAY more torquey than the original.
The best numbers I saw were around 38mpg. That's not bad, and for a 40 year old car it was amazing.