Take one fresh egg.....Carefully tape it to the accelerator pedal. Never press hard enough to break the egg. Seriously, I average 20-21 overall and 25 to 26 hwy mpg on a 1991 300SL. I understand that the 500SL does about 3 mpg less. That's not bad for a car of this weight and vintage. They weren't built for economy. In 1992 your car cost about $100K. Gas was less than $1.50 per gallon. Who'd spend that much on the car and then worry about mileage?

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Resistance is Futile.