Yes, there is a small RPM loss at the torque convertor.

The engine CANNOT exceed ~3400 RPM, as that is where the mechanical governor shuts it down. It may sound like it's wound tight as all get out, but it can actually live at that RPM all day long.

Swapping to the E4OD transmission will do 2 things. 1.) Adds an OD gear, which will lower RPM at cruise speeds. 2.) Adds a locking torque convertor, which stops the minor RPM loss caused by the C6 non-locking convertor.
However, these transmissions are not cheap, and require a controller in order to work.

If you really want to lower the highway RPM, you can increase the rear gear ratio. A 3.08 would be about as tall as you would really want to go. Any higher, and it would most likely kill both the power and MPG.
Back when the 6.9L was installed in F250/F350 trucks, they offered 3.08, 3.55, and 4.10 rear gears, and used tires in 33" tall range. The 4.10 trucks had the best towing/pulling capacity, and the 3.08 trucks had the highest top speed and unloaded MPG. The 3.55 trucks were a happy medium, giving decent MPG and pulling power.

 

One thing I forgot to mention. This engine is known to be rather loud, so Ford added a lot more sound deadening on the trucks that had it factory installed.
Just the sound of the engine's air intake is rather loud by itself. Add the diesel rattle, and it quickly gets noisy under the hood, even at 1000-1500 RPM.

 

I am not good at geography, but New Zealand is a island correct? What are you doing that you need to drive so fast? I figure on a island, you would not need to do anything but putt around at a lower speed.

My truck originaly had the diesel, the c6, and 4.10's. It was excellant for running around town and smaller roads. It just wasn't cut out for the high speed interstates though.

 

Dave, Dave

We're the same size as Colorado.....just enough room to get into top gear

 

For these old diesels, yep that's about what it takes.

 

LOL... Thats a cheap shot at New Zealand!! LOL.. Yeah, we are a 2 big islands, and a few little ones. I live in a Place called Taranaki.
Its about 4 hours drive to our countries biggest city, Auckland, and 3 and a half hours to our capital city, wellington.
it not a hick country, with goat track roads, and 1 mile from coast to coast. Yes, we can get into top gear, and stay there long enough.
Hey, let's be honest, our south island beaches were long enough for Burt Munroe many yeah ago to perfect his 1920's indian Motorbike, before coming over to your salt flats and laying down some long standing land speed records!!!!

LOL.

Really, the point of the thread was to find out what normal revs are for the Navistar, what cruising revs should be, and why the maths of working out the diff ratio wasnt working for me! Ive come to the conclusion that i have to get the tacho checked. see if the RPM's that im working on are actually correct.

The good news from the thread, is that it looks like a regular poster in it, looks like he if from NZ, and as there arent many of these trucks around here, it is always good to know some who has one... LOL

 

My truck does 2400-2500RPM @ 60mph. 31x10.50 15", NP435 with 1:1 4th gear (C6 3rd gear is 1:1). I have 3.50 gearing. Don't worry, if you are correct on your axle being 3.50 or 3.55, then 60mph isn't 3000+ RPM.

 

The C6 is a three-speed. It sounds like you aren't getting out of second gear. How many shifts is it making?

 

Yep... i had similar thoughts. and actually, much to the amusement of our calfs, in front of them as an audience, i drove up and down the road for about 10 min manually shifting it 1st, 2nd, 3rd, and no difference. also, if it was holding 2nd, the rev counter should be reading 3400 rpm (ish), which apparently they cant rev to. Hence why i was wondering if it had a 4 speed behind it. but a 17 bolt pan appears to mean a c6.
Nah, im leaning towards getting the rev counter check - ie see if its reading correct before digging around too much more!!!

 

Check the wires for the tach(rev counter). On the front of the engine, on top of a rounded housing, there should be a sensor threaded in with two wires coming off of it. This is the sensor for the tach, it reads the gear teeth inside the housing that powers the injection pump. These wires are known to get a bad connection where they go into the sensor. Also I believe you said this engine was a swap. The diesel takes a different tach, with different wires. The gas engine tach won't work.

 

Yes, a swap from a petrol, over to a diesel. It has a Genesis Ta-12 tacho amplifier under the hood. i believe that may feed the tacho??