2.62 gear ratio

yes, eleventh gear is direct in a 13 speed

 

Yep. In direct, running 2.64, on low pro 22.5 rubber, I am turning 1425 at 62 and 1500 at 65. My motor does it's best work both in terms of general performance and mpg, at 1425. 7.83 lifetime ECM logged fuel economy.

 

here is my take,,since I'm running 2.85 rears with a direct 15 speed,,my take off is strained untill the rears are rolling,,my axle wind-up if you are not REALLY easy on the clutch is bad enough to flip your coffee cup off the floor,,my pulling power on steep grades is only 28MPH,,I've owned less powerful trucks that pulled hills far better with 3.55 rears and a 10 speed OD AND got better MPG,,In my experience, tall gears like 2.64 rears are only good for flat ground with under 35k on deck,,although I will say with this set-up,my cruising RPMs at 55 yield me 7.0 according to my KR scanguage

 

Deep reduction on the 15 is REQUIRED to get it moving on a hill also

 

Your 15 speed deep reduction is not a s low as a 18918B, thats why you have startability issues.

 

12.7 Detroit likes 1400-1600 rpm at cruising speed. Depends on where youre running in flat lands or hills. I run the northeast with PLENTY of hills and I've got same engine, transmission, and tire size as you with 3.58's in the rears. I like to cruise 68mph at 1500rpm and it does great there. With your setup I'd go with 3.42's if your running mostly flat ground or 3.58's with the hilly roads.

 

WOW just finished all 15 pages and I know what gears my new to me truck will have and trans.. But wait I'm still lost as a DOT officer in a truck stop parking lot.
Being i'm looking for a truck for play not pay. And no rush to get anywhere along with running around like I have an empty 53 ft van behind me all the time.
So with a 12.7 DDIC IV motor in a 1994 exd 379 with 9 speed and 3.55's along with 24.5 LP rubber. Truck will be singled out and I would like to run Wide base singles on the back. Also plan on adding a back box to the 13 speed. And being I'm singling the truck would use this time to change out the rearend to one with the gears that would fit my needs the best. 99% of the time on paved or improved roads. So starting heavy loads in a field! Only looking for good MPG and still run up hills without holding up traffic.
Ran lots of miles in trucks with the 12.7's most were 9 OD's with 2.86's I think is what was in most. They were set up to again run with bulk loads that did not weight much. 53ft van full of Bathtubs for the most part was under 5000 lbs and most times 3500lbs per load. Run up any hill east or west coast on interstates in high. But gross them out at 79,000+ and they still did a good job but drop one gear for sure and at times 3 gears on the big hills.
This is going to be my retirement ride, Wife thinks I'm crazy to go with a EXD Hood. She wants to go with a newer Volvo. I told her if were running 60 MPH or lower one is just as good as the rest. And the peterbilt just looks way better ...
PEte

 

I have a 12.7L Detroit with 13 speed tranny and 3.55 rear end gears and low pro 22.5. Final drive ratio on tranny is .87 and 12th gear is 1 to 1. I run I-20 from Dallas, Tx to Louisiana speed limit is 75mph. At 65mph I’m running 1700rpm. 70 mph is 1800rpm. If I changed my rear end ratio to 2.93 would that lower my rpm? Will I still have enough pull power to pull a few hills?

 

If you change the 3.55's to 2.93's rpm will come down by about 300. About 1500 @ 70, 1400 @ 65.
Depending on your loads and engine (torque and hp), should be OK on level ground.

For hills, you can go down half a gear to 12th. That will bring the rpms up by 200. Still about 100 rpm's lower than what they are now. 1600 @ 65, 1700 at 70 mph.

If it is enough, depends on your engine, load and aerodynamics

 

Freightliner century condo with 430 Detroit. Loaded truck and trailer between 70-75 Pulling flatbed