The 95 E320 was once upon a time specified to do 18mpg City and 24mpg Highway.
If I take these two values and average them 50/50 we should get something like 21mpg, assuming the engine is in decent mechanical condition.
Therefore, 15mpg seem a bit too far. Best thing in my opinion is to pull the codes from 8 and 14.
Oldsinners better milage could be related to the more efficient 5 Speed tranny.
Kinda like the O2 sensor idea, trust me, I was measuring mine with all I had, including Oscope and did not see any abnormalies. My theory was that it was lazy and had no reference as to how fast the sensor should be.
I simply decided to replace the piece of art and viola, mine went to nice clean emmision and got about 16-17 mpg City only, damaging short trips. However, I feel guilty, did only about 2000 miles the past year. So, my mpg is absolutely no reference.

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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert