Spec'ing a truck for hauling a flatbed.

I've needed my diff lock several times. Not like you think, but twice in soft soil conditions in west Texas and New Mexico.

Doing FB, you're often on routes not designed for trucks. Pulling a load of rock last week out of PA, twice I couldn't upshift because of steep grades. I've encountered the same out west, but not so frequently.

An 18 definitely has it's place, and resale is higher. Other specs would depend on tire size and engine.

Most of the older 2.xx gear ratio trucks came with direct drive transmissions. For OD and pre 2010, I'll say 3.50ish or above and an 18 are perfect.

 

You'll want a longer wheel base to allow for front overhang. Don't go with a regular otr van setup at 220-230wb. More like 250wb if you're planning on a set back axle

 

I have a 2016 Pete 425 hp and 3.25 rears. And did nothing but Mountain driving with a flatbed in northwest for last couple years. It would've been nice to have bigger horsepower and shorter gears , but you work with what ya got . And that's what I got. for now atleast.

 

If i at 10 years experience and quite a range of things had to spec out exactly what id want, 3.36, short 22.5
18 speed preferred, 13 ok in 95 %

If all youve got is a 10 speed 3.55, itll work even in the mountains, itll just be more annoying to drive.

More gears more better, my truck is 3406e 1lw 485 hp, 1999
94, t600 (non-aerodyne) 285 wb midroof 63ish inch sleeper
3.36 rears with a 13 single over trans and 22.5 talls.

If i make it through these tires on this truck (nother 400k), i may move to 22.5 lp, but its already a hair long for bringing it closer to the ground.

 

Well I don't run it on the floor when pulling big grades. I've put it up 12% grades off road with chains on.. I've ran it up 10% grade, residential roads where the switch backs are so sharp you can see your trailer next to you...that's all at 80-90k pounds. Day in and day out.. You go easy on your equipment it goes easy on you. You put it in a low enough gear to where you let the engine work in the high rpms where the horse power is ,rather than the torque in the low rpms. In my opinion that does nothing but abuse driveline and you dont gain anything. On those big grades fully loaded, I was pulling it down in 3rd gear on a 13 speed... it ain't the best tool for the job. There's way better truck specs out there for working the jobs I was. But I did it, and would do it again if that's all I had to work with.

 

Unless you’re hauling heavy 3.55’s are too much for all 48.
I have 3.36, and just put tall 22.5’s on. 550 hp, 1950 torque and I have no issues pulling in mountains. I may not be the first up the hill, but I’m far far from the back, and I am averaging 7 mpg in a T680.
I would like to have lockers however..

 

3.55 is a great rear IMO, right now I run a truck with 3.55s and a 10 speed manual, puts me right at 1500 @ 70 mph. It all depends on where your engine is happy. Macks have a wide power band so I do ok with 445 hp and 80k. At my last job I had a W900 with a 565 Cummins, 18 speed 3.36 rears. It just wasn't happy at 1400 @ 70 mph. Rarely touched 18th gear resulting in very poor mpg. Ran a Paccar before that with 3.55s and it did a lot better

 

As mentioned
I would get no less than a 13 speed if buying new
If used and you find a deal but has a 10? Then Maybe