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A bus is actually more aero than a box truck.
And guys quit hating on NA diesel, that isn't really the problem.
A NA diesel CAN get better than a turbo varient but only if it is running in its ideal range, if its smoking a lot you obviously are not running in the ideal range.
Also the 7.3 is NOT an efficient turbo or non-turbo diesel, adding a turbo to one in my experience does not normally improve mileage. So throw the turbo always makes it more efficient out the window. That same statement was true of the 6.2, some 6.2's would do better fe wise after a turbo others were a whole lot worse. A turbo lets you burn more fuel more rapidly while using some power to force in air.
That said my 6.2 which was non-turbo got 10mpg running to the floor at 60mph pulling a 26' trailer and a 10' trailer at the same time over 10k in weight and that was with a 3 speed auto.
I know several who have a cummins that get in the 10mpg area but normally its a lot of weight or something real high. I have also encountered several ex box trucks with cummins motors that only get about 10-12mpg.
So in other words what he is DRIVING is the problem, he likely regardless of the motor is not going to exceed 12mpg without a MT transmission and extreme hypermiling with slow speeds.
A non-turbo diesel running a 22:1 compression ratio can potentially be more efficient than a 18:1 turbo'd motor if the motor is run in its efficient range.
Also my old 6.2 started a lot easier in winter than the ford and cummins versions I've encountered, likely because of its higher compression ratio.
I would love to own a properly designed naturally aspirated diesel suburban (or really any big iron naturally aspirated diesel car) of a more modern decent if such a thing existed. Pop in a 5sp manual and 30mpg craft mobile here we come. A 50mpg buick lesabre might be possible with old iron.
Cheers
Ryan
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