Sadly, the only really thrifty cars Benz makes are the diesels.

If you are getting 25 mpg, you are doing well. If you are NOT, however, I would check for

Bad coils/wires/plugs (you still have to replace even platinum plugs eventually)

dragging brakes (high hard brake pedal, jerky stops all the time?)

bad alignment (usually with excessive tire wear)

Leaking fuel system (pressure hoses)

bad O2 sensor, car running rich all the time (ditto for temp sensors, or bad wires on them!)

The C-class goes like scared rabbits, you really cannot expect econo car milage, but the do usually get fairly decent milage.

Use the high octane -- if the engine and ignition systems are working correctly, you will get both better performance AND better milage -- the ignition will be severely retarded all the time if it knocks on anything else, and while the engine won't be damaged, it will be sluggish and use MORE fuel.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!