Remember that MPG varies greatly with different fuel. Try a few tanks each from different stations near you and measure the difference. Also remember that winterized fuel (usually from mid-October through mid-April) will provide lower MPG numbers, depending on the exact blend - it tends to get worse in the middle of winter as they change the blend and the BTU content drops.
My last 5 tanks in my '87 (with the 6-cyl OM603 engine) have been 31-32mpg using the "premium" diesel from a local Sinclair truck stop. That's mostly with a 16-mile, one way, commute with a half-dozen stop lights, mostly 45-55mph. That's the best the car has ever gotten in seven years!!!
I expect it to drop back to high 20's with winter fuel. The smaller 602 motor should be capable of 34-38mpg with a light foot and GOOD fuel.
YMMV - literally.
