Not sure on the suspension, but unless that guy has a 190 diesel he isn't getting close to 40 on the freeway, my om606 will barely do 40 on its best day. Things to check are the good old reliable things to improve performance and mileage: valve adjustment, diesel purge, fuel and air filters, fuel tank strainer, and fuel leaks (especially around the injector return lines), and don't forget tire pressure, dragging brakes, and bad wheel bearings. Although with those last couple you'd likely have other symptoms. Check to see if the screw on top of the ALDA has been cranked up, that can really tear into your in-town MPG, IIRC counter clockwise increases fuel performance and fuel consupmtion, but it's best to search that to be sure.

How many miles on the car? My 85 300SD after a valve adjust and fresh fluids and filters all around will still only do 28mpg highway at best.