His - Denim Blue A220 AMG Line Premium / Hers - Obsidian Black R172 SLK55
I have an E350CDI Avantgarde with the 265hp engine and 7G-Tronic Plus gearbox. Average over the first 4,200 miles has been 39mpg (real, brim-to-brim measure). The trip computer accuracy varies anything from 2% optimistic to over 12% optimistic which is much more than it did on my W204 C220CDI, so beware of anyone who quotes their "since reset" consumption.
My driving is mainly on rural A- and B-roads, with a bit of motorway and town driving thrown in. I tend to drive briskly but smoothly, but confess to finding the performance of the V6 somewhat addictive. Once (if?) that wears off and the drivetrain has fully loosened up, I'd expect consumption to be comparable with my pre-BE W204 which averaged roughly 42mpg over 42,000 miles.
On a recent weekend away that included a 300-mile motorway run at "keeping up with the traffic" speeds plus about 50-60 miles of running around the trip computer showed 49mpg average which, with a 10% error, is 44mpg.