That sounds really low, I am getting ~21-24 on the same ratio of hw/local driving. Sounds like new plugs or coli replacement. Start with the obvious, check air pressure.

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95 E320 Coupe Black - 70,000 miles with Parchment interior. Stock through out
93 400E Dark Blue - 65,000 miles Sold to my dad
86 300E Gold(yuck!!!) 126,000 miles- Sold, a really fun car with a lot of memories