The mpg I'm getting with my 83 380sel is less than it used to be. I used to average 20+ mpg. Now I'm getting 18 - 18.5. The motor has 210,000 miles and seems to run fine (smooth idle, no missing, easy starts).

Questions:
Is this just normal for a higher mileage engine?
Does this mean my compression is slowly decreasing?
Would new plugs help (they are past due, but it's cold and the car seems to run fine)?

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126