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

ken Juul
91 300 SE
85 380 SL