I have achieved a whopping 40.06mpg with my 1986 90 running a 200tdi. That's with 235x85xr16 tyres on the standard fender transfer box. It almost seemed beyond comprehension, but it was a good run between Leeds and Filey, with an average speed of 55mph and little blasts at 65. I regularly achieve above 35mpg and it doesn't half shift too, will top at about 88mph. I found that overhauling my injectors and installing a new injection pump (eBay bargain) made a tremendous difference to power and economy.
-Tom