Owner many in the 80’s 42 was top mpg i have heard of higher but have never seen it to be true
Mine is a 2006 Jetta that get a average of 45-48 mpg.
A lighter frame car like a golf will get a bit more, but not much, and if you swap the computer chip you can get more.
Its been years since I’ve seen one, (I assume that you’re talking about the diesel Rabbit pick up offered during the 1980’s) but remember them well. If you’ve found one that isn’t consumed by rust, get one with a manual transmission, the automatic is so s-l-o-w that even late model stock type II will pass you up.
To answer your question, somewhere between 20-30 to in city driving is about right (range is because it depends upon load and terrain), upwards to low 40’s on the highway is possible. Keep in mind that there isn’t much BHP to begin with, if my memory is still good (no guarantees on that), it has between 48 to 52 BHP. So, you can see that its performance is limited.
Models with the a/c had a tendency to vibrate the dash and rattle a bit at idle with the compressor engaged; if you spend a little time adjusting and positioning the engine with the a/c on at idle you can reduce the vibration greatly. Don’t be surprised if it needs all the motor/transmission mounts, they had a history of tearing up mounts especially the round one on the (right?) side. When those gave up the ghost, the engine would drop down about 1" or so and rest on the metal frame of the mount increasing the vibration greatly.
If you get one and the a/c is working, I’d recommend a device called the Passmaster, which if you set it right, will dis-engage the a/c compressor clutch at 3/4 throttle and above (some people set up so that it would only work at WOT (wide open throttle). This will help your acceleration greatly if the a/c is on.
Hope this helps, a car nut.