In my 1991 420SEL with 192K miles, the economy gauge rests comfortably all the way over to the left at idle. It moves slightly to the right when the A/C compressor is on. When I was having problems with the idle (sticky exhaust valve) the needle regularly moved one-quarter of an inch to the right during idle. Since the idle problem was fixed and the valve adjusted, the needle stays to the left. I don't know if this is true for all M-Bs or whether your model is a little different from the V8s. The fact that you can get a better reading at highway speed may be relevant, however.
1989 300TE "Alice"
1990 300CE "Sam Spade"
1991 300CE "Beowulf" RIP (06.1991 - 10.10.2007)
1998 E320 "Orson"
2002 C320 Wagon "Molly Fox"
Res non semper sunt quae esse videntur
Not in this weather!