I am following this thread with great interest. If the dealer is de-tuning the engine to meet some arbitrary govt regulations I’ll get my GLC serviced elsewhere. That is a significant (huge) drop in efficiency. I too am a nerd regarding how I drive and the average mpg per tank. If I had that kind of drop I would notice. My MPG is consistent all year around from sub freezing weeks in winter to 90 degree F weeks in summer.I do the same journey repeatedly. 2 years ago my 250d would manage mid-late 40's. After it's service incl software update i noticed that fell to 40-42. After this years service I suspect it has dropped again, but had my roof-bars on, so need to check without.