change rear end ratio or change transmission

First of all, I have exactly the same setup you have except my 550 is factory and not a turned up 475. There is a big big difference, and I have never been a fan of a turned up 475. Don’t ask me what the difference is because I couldn’t tell you. I very seldom have to drop a gear going up a hill. I’ve said this more than once, I want power and couldn’t care less about speed. Second, you haul cattle and hay, both are heavy and wind grabbing, to drop one gear going up a hill, I would call respectable. It’s your truck and you call the shots, but I would leave it alone. Best part of $.05 worth is it didn’t cost you anything.

 

Bet OP ended up with the crappy low-NOx file when he uprated. The good 550 file would have been in it if it was a 550 from factory or if it was uprated prior to the low-NOx file being released.

 

I dont understand why you are topped out at 72 mph and 1650 rpm? And you said there were no limiters set.

I also 3:70s, 18 speed and lp 22.5s. I can run 85mph all day if I want. Just gonna be drinking fuel..

3:70s will climb a hill better that 3:36s

 

You have that backwards.

HP equals speed.
Torque is a turning twisting applied force. Torque equals Work.

 

Horsepower, by definition, is the speed at which work is performed.

Horsepower = (Force × Distance) ÷ Time

1 horsepower is the ability to move 33,000 lbs 1 foot in 1 minute.

 

First sentence is correct.

 

Changing the transmission won’t make a difference. I think 13 and 18 Double overdrives are all .86, and .73 in top two gears. 370s are going to pull a hill easier than anything lower. The set up you have is good for hills. The problem if any is higher RPMs, maybe 1700-1750 are needed, at the bottom of the hill. I agree with with what’s already been said.Maybe Top Speed or RPMS are limited. Even with 370s Truck should run well over 90 mph. If it’s not limited, maybe theres a fuel or turbo boost problem. Leaking charge air cooler. Somethings not right. Check the simple stuff first. One of the real Mechanics on here can help you figure it out.

 

sounds more like a junk file in ecm that 550 should darn near hit 100mph even with 370 rear or have your boss turn top speed up

 

3.25 will put you exactly 1/2 gear higher than 3.70.
You’d be running the same rpm at the same speed in 17th gear as you do now in 18th. I’d be like adding a whole (1/2) gear.
My truck has 2.93 gears and I love it! I just run around in 17th gear all day long (unless I want to go 80mph) it’s essentially the same as having 3.42.

 

That's what Bruce Mallinson down at Pittsburgh Power says to do. Run high rear gears in direct drive on the trans, not O/D, and the truck will pull better. He explains it better than I can, but it has something to do with torque reduction through gearing in O/D.