Here's my (recent) experience -
I bought my 96E320 last summer, with just under 50k miles. It had been driven very little the prior two or so years (100mi/mo ? +/-).

I took several driving/sightseeing 1000 mi. roundtrips, getting used to the car (and it to me). The highway mileage, using CC @ 74-75 mph, was consistently 25mpg...right on the EPA hwy. estimate.

I recently had an independent shop which had been maintaining the car for the prior owner perform the recommended 60k mi. service, which included the installation of new plugs which I'd already bought - the Bosch double electrode plugs.

I've taken one 600 mi. R/T since that servicing and got just over 27mpg for the trip. (I also had the formerly rather noisy rear Michelin XGT's shaved just before the trip...waht a difference in ride and noise!)

So, I don't know if the new plugs are responsible for the rather dramatic increase in hwy mileage, but it would seem so.

Opinions?