Inflating your tires to a higher psi will get you a few more mpg, as well as a bumpier ride. I've heard 34psi in the front and 36psi in the back will get you a little better mpg. Sometimes, a leaky fuel fill cap can lead to worse mpg. It may need replacing.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.