ML 63 MPG...
The basic breakdown with some playing was this:
Over a decent stretch of I5 I was able to get this up (and over) 25 MPG for quite some time (at least half an hour) With some clever drafting (tailgating) and very smooth, manual throttle with as little turning as possible. It seems the best fuel economy comes around the 65ish MPH. As you go faster it seems to peel off, and around 75-80 MPH it peels off hard back towards that 15 MPG. Interestingly, if you go slower (say, 55 MPH), your fuel economy worsens also -- you are burning less gas, but you're burning it for longer to get the same distance!
Here's a fun camera-phone photo partway through the experiment:
Same with my ML55, If I run below 75, it'll jump to low 20s, high teens. Above that I am just pushing air like a brick!! The reason why it does real well above 55 MPH to about 75 MPH is engine efficiency. Above 75 MPH, you're just pushing a whole lot of air on both the 55 and the 63.



