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

Average mileage drop in cold weather, 10 - 15 %, more, if you're pushing snow around.
If it starts & runs well, I would treat it to a new set of plugs in the spring ( perfect time to do a compression test as well.

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2007 C 230 Sport.

I should have noted that I've owned the car from 60k miles. The 20+ mpg average lasted until about 175k. Since then it has dropped.

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

How old is the oxygen sensor?

Sixto
95 S420
87 300SDL

given the mileage, i would go through everything on it. replace plugs, cap, rotor, wires, injectors, seals and holders. have the intake smoked with a machine to look for vacuum leaks and check the fuel pressures to see if the control pressure is dropping off. also, it may benefit from a motorvac, after everything is done. the bottom ends are bullet proof and rarely cause running problems. i.e., compression loss, unless the oil changes have been lax. good luck, chuck.

sixto - good question - I don't know if I ever replaced it. I can't remember what part of the exhaust it's on, but it may have been replaced when an exhaust part was changed (I'll have to look at old receipts). I've had the catylitic converter replaced and also had a timevalve stainless system installed. Can the sensor be checked on the car (how)?

porkface - I'll be checking / replacing the electrical components when it gets a little warmer. I would think that if any of those were really bad (enough to drop the mpg significantly) I would have a rougher running engine. I'll also check for vacuum leaks. After that, if performance hasn't improved, I'll ask my shop about checking fuel pressures etc. I change oil every 5k with dino and it doesn't use much between changes (maybe a quart or two).

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

as for the running, the changes might be slight so you wouldn't notice. had an 86 560 that ran well, but the exhaust didn't smell right. put it on the ignition scope and the firing didn't look right. put a set of wires on it and, voila', it straightened right out. customer never complained about a running problem, but was really suprised at the difference. a set of injectors will have the same effect. i love putting new injectors in kjet cars, they really wake up. good luck, chuck.