I recently got 28mpg on my '89 740Turbo wagon that has previously run at 24mpg! I don't know what to attribute the increase to. Here's what changed: On Friday I did some work and put in:
(1) new exhaust system starting from the cat and ending in the gooseneck over the rear axle. I left off the rear muffler. The new cat has a hole (for an oxygen sensor) that is temporarily unplugged and bleeds exhaust upstream of the cat. The old cat was rotten and had broken apart internally.
(2) new fuel pressure regulator mounted on the injector rail
(3) 12.5 gallons of 87 octane unleaded from a 76 gas station.
(4) FYI: oil in the engine is 10W-30
(5) removed the stock rooftop luggage rack and plugged the holes
On Saturday I drove 280 miles at 78 mph, with some hills. I put in about 10 gallons of gas to top-up the tank again. This calculates to 28 mpg! I've never had mileage even slightly above 24mpg.

What caused the increase? New unclogged cat? upstream exhaust leak at the oxygen sensor port? Lack of drag from the luggage rack?

--Dan
Seattle, WA
'89 740-T Wagon