This is a relevant topic for me as well. I think the 26.4 US gal capacity claimed by MB is inaccurate, for whatever reason. Last month I stuck in 26.5? Gals and from my experience with my fuel gauge(which I admit is quite subjective) I did not coast in to the pump on fumes. I think it holds over 27 gals. As sailquik points out, filling it to brim adds alot more also, even though I think it has a short filler tube it is worth something. I have found that if I fill the last gal or 2 very slowly I can get almost 1 gal more in than if I go full throttle. By filling slowly near the end I assume it lets as much air as possible burp out of the tank. Also, I am trying to avoid blasting off a vent line with too much pressure. I would recommend this "max fill" method only if you are jumping right back on the freeway due to the possibility of venting or leaking out of one of the points on top of the tank that we see discussed often.
The bigger issue all of this variance in tank topping off raises is, trying to accurately determine fuel mileage. One tank I get 27mpg, the next 24, hmm. Then I remember I just removed the fuel nozzle when it auto-clicked off when I filled it for my tank that got 24mpg, and didnt trick an extra gallon into it like I did to get 27. Thus the variance we see when we compare our mpg as a group.
Just my opinion, at least.