Had this exact problem on my '86 with 12mpg and black smoke (unburned fuel)
upon acceleration. The shield braid on the O2 wire actually cut thru the
inner layer of insulation due to years of vibration. After fixing this, it
jumped back up to 18mpg.

Wish I could get 22mpg, then I wouldn't have to think about a second
vehicle. Any thoughts on diesel powered late model VW sedans? i.e. Golf vs.
Jetta vs. New Beetle?