question(s) for the Mechanics

3.55 revolutions

 

I would not rely on what they tell you.

Crawl under the truck and somewhere on the housing there should be a tag.

Get a rag with thinner or a soft wire brush and clean up the tag and that should tell you what it is.

If you can't figure it out snap a photo of it and someone here will be able to help you.

If they are 3:55 Rears that's pretty steep for a 300 hp engine.

Rears like that make a smaller horsepower engine pull Hills more slowly.

Lower gearing gives you more power especially with the smaller engine.

My Rears are 3:90s, and my truck pulls Hills really well. Perfect gearing actually for my engine and my truck and what I do.

Honestly it would surprise me if they are 355 Rears. Back then they tended to gear everything much lower.

Like I said anything is possible but I would think they would be at least 3:90 or 4:11 Rears maybe even 4:33 to be honest.

When you get time check it out and let us know what you have.

 

He could jack it up in the air and mark the yoke and tire and make one full revolution of the tire while counting the revolutions of the yoke. That method never lies.

 

Don’t always rely on the tag with something that old. My ‘67 came with 4:33’s with the Ntc 335 and still has the tag’s for them on the original housings. It’s had 4.11’s in it since the ntc 350 was put in back in ‘92. I also ended up with 3.55’s out of a ‘85 FL Coe parts truck that were still tagged and truck vin’d for 3:90’s. I was hoping for the 3.90’s to put behind the nh250 in my ‘72 White that had 4:88’s in it.

 

3:90’s and 4:11’s work the best with the NTC’s and 13 speeds. 4.33’s work great if you have the extra gears of an aux box. 3.55’s work ok for flat land but you will be slow climbing hills.

 

That's unusual that truck would be speccd like that.

Those rears would make the truck seems sluggish and especially slow on Hills. And even more pronounced with a lower horsepower engine.

If the engine runs good and you want to run it as it is for however long I would suggest looking around for a set of 390 Rears. Or 4:11 or 4:33 if you don't mind it being geared that low.

If it were me I would look for a nice set of Freightliner air ride Rears that you could put up under there and you'll improve your ability to climb and you will improve the ride quality of the truck.

You can go on to the road ranger site and look at the road speed calculator and that will tell you what RPM's you will be at with whatever transmission you have and tire size.

Being geared lower is going to make you have more RPMs at highway speeds.

For me and my big cam 390 rears are perfect. At 65 miles an hour in my truck is turning about 1550 RPMs with 22.5 tires and .73 od and I have plenty of power to climb Hills.

 

3.55’s back in the 55 mph era when that was built woulda kept the cruise rpms way to low and not in the best power band for that engine. Pulling top end woulda been lazy.
Maybe some fleets were starting to experiment that far back with higher rear gears. O/O’s wouldn’t have bothered with that back then I don’t believe.

 

I know. Cruise rpm is the rpm engine is normally running at a steady speed.