So I am not crazy thanks.
Back when I bought my 2015 ML 250 Bluetec it was listed as having 18" rims which I wanted , but they turned out to be 19's which the MB salesman termed "an upgrade". I bought the car and love it , but after the first 35K miles or so I bought fatter tires, going from 50 series to 55 series and the ride is noticeably "softer" over NJ's numerous potholes and bumps etc. But the MB dealer thought I committed a mortal sin. The tire diameter went from 29" to 29.5 ". I don't know what that does to the MPG but the ride is more forgiving and the pep seems unaffected.
Here's to fat....tires!
jz