Running a flatland dedicate run, where should I look for fuel efficiency?

So I actually hopped out of the driver's seat after I broke my neck working in the oil fields at the beginning of this year and don't own any equipment. A friend of mine badgers me every month or so to come and drive for him and I've always told him no but he managed to come up with a dedicated run that takes all of 7 hours every day to complete and is done entirely on flat pavement hauling a single two axle trailer.

Even with a rental truck, he's offered me rates that are high enough that I will very easily be able to draw enough of a profit for the first few months to establish a real stash of working capital that could put me into a position to get into a proper lease.

I'm curious to know if any of you have recommendations as to what I should look for in a rental. I have a few lines on possibilities here and have decided that because it's a rental and because it's flat land all the way, fuel economy will be my absolute primary concern. For this reason, I'm looking at a gutless mercedes engine in something like a Freightliner or something.

So again, any recommendations? Anyone have any experience with fuel economy under these conditions with a Volvo D13 or D11? I'm not looking for power as I'll be doing battle with an even grade all the way to my destination and back but am required to have a "newer" truck.

Specifically interested in Volvo, Mercedes and Detroit engines on this one but would love to hear about most anything else as well.

 

Considering that all I'm hauling is a dry van, tandem axle, no I won't be looking for specific equipment. I'll want some creature comforts in the cab but aside from that, I'm not looking for anything that effects the suspension or what not. Gear ratios may be important to maximize fuel economy but with rentals, this kind of stuff is a little too specific to request generally. Engine varieties are easier.

 

I appreciate the recommendations for older model engines and trucks but again, I'm looking for input in respect to newer engines. In regard to having to worry about repairs, I'm going on a full service rental so if a Mercedes gets great mileage but blows up, all that concerns me even in that situation is that the engine got great mileage before I gave it back to the rental company.

Maintenance is NOT, I repeat NOT a concern here, only fuel mileage is. Even in respect to dependability, I will be able to snag a second rental truck for no extra charge as long as the one I'm using is being repaired.

A rental company out here in my neck of the woods recently told me that they rent all Western Star day cabs with DD15s in them. Any input on the DD15 500HP engine?