I have an M2 with a 17'10" box (quite small) and I'll share some of my thoughts...
The rig has the MBE926 engine with 300HP and 860 lb-ft torque. We've towed a Honda CR-V and the rig barely even knows it's back there. From what I've read and been told, the fuel mileage depends more on the total weight than on the engine specs. In other words, it takes X horsepower to move the rig and Y gallons of fuel to produce that horsepower. So, my rig might be working a bit harder to go 65 mph than a rig with a larger engine but roughly the same amount of fuel is consumed. We have 10k miles on the rig and are averaging about 9.8 mpg. I'm hoping to get over 10 as it gets even more broken in.
More info...
As delivered, the M2 did ride somewhat harsh. I attribute this mainly to the underloaded front axle (8k pounds on a 12k axle). Adding Donvel air stabilizers (air springs) made a world of difference! I don't have any experience with other rigs so I can't say whether the M2 is any better or worse.
Regarding 'enough power'... There are very, very few occasions where I wish for a bit more power. Overall, it pulls the hills at about 55-60 mph. Getting a running start helps a lot (it doesn't accelerate uphill well at all). For the most part, we're limited by the curves in the road and not the power to pull the grade. (In other words, we have to slow down for the turns before we run out of power to climb.)
There are several 'engineering' approaches to help you decide. There's an Excel spreadsheet that will tell you the amount of HP required to pull a certain weight up a certain grade. The truck dealers can also run a 'profile' for you that gives you almost a perfect picture of your expected performance. I had the Allison transmission rep run a profile for us. With this information we were able to select an axle ratio to optimize performance for 65 mph and knew that we could pull 25k pounds up a 7% grade at about 50 mph.
Hope this helps.
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Regards,
Alan
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