Reading this thread brought back some fond (and not so fond) memories of my '82 diesel Jetta.
I routinely got 48-49mpg over mostly highway driving with an occasional 52 mpg when I was on vacation really doing highway driving. I noticed that if the fuel mileage dropped a couple of mpg's it's because I had a rough week at work.
I wasn't using ANY hyper-miling technique unless right foot to the firewall is one of them
Problems? I had a few costly ones which I figured out too late was a dealer induced problem. The glow plug relay ($$) had to be replaced several times before the dealer discovered the relay block ($$$$) and all the relays were severely corroded. They discovered the antenna lead which came through the left fender well was draped over the relay block. The antenna lead was supposed to have a grommet to stop any wheel well spray from running down the lead and dripping over the relay block. Of course it wasn't there and much later I remembered or found out somehow that when I bought the car (excuse me, "It's not a car, it's a Volkswagen" slogan), the antenna and original radio were dealer installed, not by the factory in Germany. Moral of the story, check the antenna lead entry point into the passenger cabin.
Also, be VERY gentle with the window crank AND maybe buy a prophylactic spare. It's a pain to roll the window down (or up) without it. Keep some vice grips handy. When I went to get a new crank, the dealer's new parts dept had a whole rack of them in the open so you didn't even have to ask at the window. Draw your own conclusions.
My dash would vibrate wildly when I started up the engine cold. Periodically check for loose screws in the dash.
The odometer died on mine and through a motorcycle friend I found out VDO odometers were notorious for that and that there was a shop in the Los Angeles area that had a full time business of repairing them.
Aside from the problems, I have some fond memories of trips I took on vacations with my Jetta. The huge trunk and back seat area held a lot camping gear.
A tip: I kept a pair of big rubber gloves in the trunk that I would don before I fueled up. I didn't care to smell like a diesel mechanic.
-- Teri