O2 sensor has only close to 11,XXX miles on it. Spark plugs are about the same as well as air filter. No CEL code. Nothing was tampered with. Fuel/Air mixture is as it was when car came from the factory. I usualy would get anywhere from 280-300 miles on a full tank of gas (city and highway respectively) now I only get 240 miles city. Any suggestions

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1990 190E 3.0L