spec'ing a shed hauler - axle ratio etc

[jmartin]

My brother is getting quotes on a new truck that will be set up for shed hauling, think moving Amish barns.... He has asked me to get feedback on the specifications we're looking at so far.

Shed hauling is a unique application, some points for consideration:

-50k miles per year driven typically
-Fuel economy is not a primary concern
-Up to 50% of miles driven are unloaded, up to 70 mph
-Most loaded miles gross around 35k, 58-65 mph
-Some loaded miles (up to 16 ft wide) require full engine power, especially on grades (high air resistance)
-Custom built trailers, Pine Hill bumper pull in our case
-No payload on truck, other than the shed Mule (google Cardinal Mule)
-Must be reliable for many years, with good upkeep

Many, probably most, shed haulers run pickup trucks, with a few using single axle semi tractors. The advantage of a medium duty is being able to spec a much heavier engine and transmission than a pickup, and still have a low-ish ground to cab floor height. The operator is jumping in and out of the the truck many times per day, which takes a greater toll on knees and back with a higher cab. Pickup trucks are pushed to the extreme in these conditions, leading to a shorter lifespan, and expensive maintenance.

After getting initial quotes from Navistar, Freightliner, and Hino, we are leaning toward the International MV607 medium duty set up with these specs:

-Cummins L9 350 with 1050 lb ft rating, with Jake brake
-Allison 3000 RDS Close Ratio, 6-speed with double overdrive, no retarder, Performance programming
-10R22.5 rear tires
-255/70R22.5 front tires
-Disc brakes
-Locking diff

Biggest concern is to make sure we are not lugging or otherwise abusing the engine, assuming the L9 can suffer from the same issues as the X15 when lugged constantly. Among the available axle ratios are 3.55, 3.70, 3.90, 4.11, and 4.33. According to what I can locate online, the Allison 3000 is 1:1 in 4th gear, 0.74:1 in 5th, and 0.65:1 in 6th. I presume Allison's "performance programming" option may help avoid lugging, but have not found any solid information on exactly what it does.

Based on given tire size, available axle ratios, Allison gearing, and typical operating conditions given in the list above, what is the best ratio to avoid lugging under any conditions, while still running efficiently at 70 or 75 mph empty? I'm leaning toward 3.90 but want to see what the experts say.