Ok, two different topics, first, fuel mileage.

Unless you know how miles you went and on how many gallons of gas, you're just 'whistling in the wind'. The terms 1/2 or 3/4 of a tank mean nothing, you will probably find that you will get a LOT more miles out 1/4 than another.

On my Range Rover for instance, I get nearly double the miles out of the last 1/4 tank than I do out of the first 1/4 according to the gauge.

Next, tire pressure. Ideally the tires should be inflated to the point where the tread is in uniform contact with the road side to side. Too much will mean the tire is only touching the road on a small portion of the middle of the tread, too little will mean the tire is resting on the shoulders only, with the middle raised up clear of the road.

In either under or over-inflated situations, you have less than the proper amount of tire in contact with the road. That means you will also have less traction, steering and braking.

A few pounds more or less than what the manufacturer recommends in the manual won't do anything good or bad to the mileage. Go beyond a 'few pounds' and you will have differences, none of them good.

 

Try different gas stations
i know my ML hate Race Track,7/11 and HESS station
mobil1 and BP not too bad
and it loves SHELL stations

 

What kind of mileage (MPG based on experience) should these ML320's get?

 

Not much, these vehicles are pretty heavy and have a lot of windage. My wife's ML320 is right about the EPA estimate, 15 MPG city and about 18 MPG highway, as long as you don't step on it. I have a 1999 E320 that is not quite ready to be put on the road yet (needs front fenders) and I wander if it will be similar, as even though the E320 is much lighter, they share the same engine. My BMW 528iT is about the same size, but it does great on the highway, averaging 25.5 MPG even when driven around 110 KPH.