1995 E320 terrible gas mileage

[okto]

Recently replaced the plugs on my M104. Expected to see an improvement in gas mileage, since the old ones were 12kmi old, gapped a mile wide, and the wrong type of plug (resistors).
Just did my first fill up since the plug change:

15.4mpg average, about 50/50 highway city! (My first thought was that I should have sprung for an E420 since I wouldn't have lost anything in efficiency!)

OBSERVATIONS:
> She purrs like a kitten and pulls like a train with no stumbles or stalls.
> The idle is a liiittle rough, but mild enough that I can just barely feel it through the wheel. Coming from a diesel I have no meaningful baseline for expected idle vibration.
> Always catches on the second rotation, no surging at idle.
> RPMs start at 1k and ease down smoothly to around 500 within a second or two of startup in 60-70 degree ambients.
> There used to be a gasoline smell from the engine bay and tailpipe for a few seconds after startup, have not observed this since spark plug change.


This is my first gasser, so I don't really know where to start. I'm trying to ask as many questions as I can to get an idea of what to expect. Previously drove an E300D, and the world of spark plugs and throttles is entirely new to me.