re: fuel economy
Any A/C work now days is expensive because of the EPA requirements. Get a couple of quotes from different A/C shops and good luck.
A good tune up, plugs, wires, cap/rotor never hurts gas mileage.
don't forget the air filter.
Tire pressure, keep at the recommended level, check often.
slipping transmission? had a fluid/filter change recently?
dragging brakes, do you go through pads more quickly than you would expect?
how does the inside of the tail pipe look. A coating of black soot would indicate running rich. Any number of components in the injection system could cause that.
How about a leak in the fuel system. do you smell gas outside the car after it has sat a few minutes after driving.
Off the top of my head these are a few things to check.
PS. I generally fill up as the needle approaches the top of the red. 450 miles is theoretical, haven't seen if it is attaibable. good Luck
91 300 SE
85 380 SL