Gear Ratio Confusion

Well it will be hard to get fuel economy at those kind of speeds.
I have a DD15 and it seems to like to run a little higher rpm than what the mfg says. It pulls better above 1,450 rpm and kind of seems to fall off by 1,700 rpm.
What tire size you run?
Assuming 11R22.5, 3.55 rears would run about 1,600 rpm at 75 mph or 1,700 rpm at 80 mph.
For a DD13 that might be about right to have any kind of performance. For fuel economy you'd want to be under 1,500 rpm, so say a 3.36 rear but it would be a real dog.
I had a 450 Mercedes with 3.58 and it had no guts at all. I changed them to 4.11 and it pulled way better and gained almost a full mpg, but I only ran it at 62 mph.

 

4.10 should pull a small elephant up the side of building.

2.64 will give you the speed you want, but you'd want a 13 or 18 for the hills - I imagine a 10 you'd be forever shifting 9/10.

Follow @Cowpie1 - he went 2.64/13 or 18. I picked his brain a while back on this subject.

3.55/13 with my N14 gives 1500rpm@ 69mph. Gets 7 mpg.

 

I drove a '99 Frightliner Columbia with a 500 Detroit, 13, and 3:55's, and that was a good combination, I thought. 7mpg, all day and 70 mph was like 1550r's. While I didn't care for the truck all that much, ( it did ride nice) if I got another truck, ( did Hell freeze over yet?) it would have that setup.

 

2.64 with 10 speed direct & 22.5 LP:
1300-1700 rpm
6th: 18-23mph
7th: 24-31mph
8th: 32-42mph
9th: 44-57mph
10th: 59-77mph


2.64 with 13 speed double-over & 22.5 LP:
8th: 36-48
9th: 43-56
10th: 50-66
11th: 59-77
12th: 68-90
13th: 81-105


3.55 with 13 speed DO & 22.5lp:
8th 27-35
9th 32-42
10th 37-49
11th 44-57
12th 51-67
13th 60-78


3.36 with 13 speed DO & 22.5lp:
8th 29-37
9th 34-44
10th 40-52
11th 46-61
12th 54-70
13th 63-83

11R22.5 & 24.5LP is ~4% faster
11R24.5 is ~8% faster

 

The DD series is probably about the best out there today but it still doesn't feel like a pre egr ddec3 did. In our dd15 455 I would have volvo d11's and old R model mack's out gun me on the hills. We also had de-tuned ryder trucks so I have no idea if they were actually 455 or not. My dad swears up and down his previous 460 e7 would walk a new dd

 

From my experience, Schneider usually hauls light loads ( out of Green Bay, anyway) toilet paper, diapers, paper towels, and I think it was their way to drop the rpms for fuel mileage, and those trucks don't downshift much and they are governed at 62, I'm sure. That 2:64 ratio is a bit odd, not many others use that ( I don't think) Like I say, I'd go with mid 3's and a big motor. Years ago, with no hp, you had to wind the piss out of those motors, and 4:11, even 4:30 gears were normal.

 

My truck is a 2.64 with direct 10. I don't constantly shift or have issues with power. I get a light load once in a while but usually gross 72k or higher. This puts my cruise speed at 60 mph. If you want a faster truck than that you need a faster rear or an OD Trans with a more conventional rear. You can find calculators online to do the work for you then work on finding the truck that matches your needs. Rears themselves don't determine pulling power; they determine the RPM at a particular road speed and that determines the HP you will be putting out or "pulling power."

 

They're not the only one, they were just one of the first. Lots of new fleet trucks run 2.50's now, and there are even lower options.

When paired with a 10 speed direct, 2.64's are a good fuel mileage gear for the 12.7 running ~62. It'll comfortably run 57-68, with 2.79's being better for 55mph and 2.50 being better for 70+


You probably won't find an unmodified truck with a 13 speed and that type of gearing though, and if you wanted to go custom there is always the 13 speed direct and the 13 speed single over-drive. The main downside is that these transmissions are not rated to handle as much torque as the standard 13 double-over.