43.5 MPG average since the last service. I don't baby it, but I let DrivePilot drive most of the time.
2018 Mercedes E300 factory-custom mutt. E-Class chassis, S-Class interior and electronics, C-Class powertrain. The car still shows the S-Class graphic on some of the screens because of it. Full Bosch Mobility Solutions suite, minus night-vision due to limitations of the headlights. Some special sauce. All the bells & whistles, including ones not normally available. Interior, suspension, seats, insulation, etc. It's very much built for my individual needs and desires. Car is heavier than stock because of this. It also sits 1.5" higher than standard because I wanted extra ground clearance for parking blocks, water, etc. Seats have extra head padding and coverage due to health issues.
The pisser is that this car was supposed to get 85-95 real-world MPG (not that MPGe garbage), but the US EPA blocked that and said I should buy the USA-approved model which got a whole 25MPG. The EPA insisted that was "cleaner and better for the environment" than the powertrain I was trying to import... nevermind that it puts out far more CO, CO2, and soot than the powertrain I wanted. They also wouldn't let me put in the more efficient HVAC system either. That's when I knew these guys weren't serious about protecting the environment.
I managed to get the no-glare headlights physically installed, but the clipboard warriors said I could only run them at full-bright or dipped-beam because of some law passed some 56 years ago, so they run in a quasi-automatic failsafe mode. 84 individually-controllable LEDs per side(!). In theory you can draw things on the road with it.
Fortunately, the people I worked with to get this car made took pity on me and did the best with what we were allowed to bring in. Through their efforts, that's why I'm getting the 43.5 MPG average from this car. Totally non-EPA-compliant, but 45+MPG vs. 25 MPG seems to make a lot more sense to me, both economically and environmentally. Still a far cry from 85MPG.