Mikey D

Howdy Folks,

I am a relative noob to this site and have to thank all of the people who have helped with my questions. Please excuse me if I ask ignorant questions or supply redundant information as I am just learning about this.

Lifer suggested posting in this forum as ya'll are the towing gurus...

I have purchased what I hope to be my dream truck. It is a '95 Freightliner with a 5.9 - 12 Valve & a P-pump. The gentleman I purchased it from said this engine (190 hp by the sticker) redlines at 2600. He however installed a 3000 rpm spring kit in the pump. He sounded quite knowledgeable about these engines and claimed to have built several high hp ones. I have a pretty sensitive BS detector and he seemed for real.

My goal is to turn this unit into a motorhome/toy-box hauler/non-commercial service truck.

It weighs 13K now and I plan on adding another 2-3K for the motorhome/service truck mods. The sticker on the truck says it has a 33K GVW and I plan on registering it non-commercial GVW 25,999. My trailer maxes out at around 10K fully loaded.

I though I was running rpm's that are way too high on the freeway. Engine rpm’s are as follows:

It tach's;

2500 @ 70 mph

and

2200 @ 60 mph

However after pereusing this site (and looking through all of the threads that popped up after searching rpm in this forum) I may be OK as it is. My goal is to be able to run 70-80 mph on the freeway and be able to go up grades at speed. I have made arrangements with a local rear-end surgeon to swap the 4.33 ring & pinion for a 3.42 set. This will decrease my 70 mph rpm to 2K which I was convinced would be beneficial. Any comment or opinions are welcome and encouraged.

The truck has an Allison 4 speed auto whose 4th gear is a 1:1 ratio and 41" diameter tires

I have posted a new thread because I have not found an old one whose vehicle had these specifications. If I have double posted, please excuse me and please point me in the correct direction.

Here is the link to the first thread pertaining to this question (and a pict of the front of the truck):

https://www.dieseltruckresource.com/...e-t259958.html

Thank you for your time,

Mikey

HOHN

The RPM might be a little high. But if you are going to load it heavy, keep the cruise RPM above 1900 or so (per the ISB gearing recommendation).

It's shoot for between 2200 and 2400 for a hi-speed cruise.

JMO

Russ Roth

You might check to see if your Allison might have another gear that is locked out. I drove one I was told by the people that did the specing that was the case. That would lower your RPM if it had a locked out OD. It was a Fire truck that the FD wanted to not go over 62 MPH. They didn't like it if you went faster. DAMHIK.

Mikey D

Thank you Russ.

The first place I took the truck was my local tranny shop who gave it a once over. There are only 4 gears and no hope of changing to an OD. On the bright side, they said it was in GREAT shape!

Thanks again!

Mikey

jkitterman

You should not change the gearing or it will tow terrible. You want to aim for 2150-2250 rpm at a cruise of 65. This is per Cummins specing tool. Since you only have a few gears, you will have rather large steps if you regear to 3.42. If you really want to run faster, you should look into a repower with a larger engine or a different vehicle.

Jaxom

2 speed rear end would be the ticket.

linetrash75

I would think the 3.42 would get you to the 2100-2200 rpm at 70, ideal for cruising. Just don't expect 1st gear to thought back in the seat. You won't notice any power differnce in the rest of the gears. Would look into an under drive thought if you plan on towwing in hilly country.

Mikey D

Ouch!

Mikey D

I have looked into that or the "magic box" manual switching overdrive unit. This is a commercial (big rig) Rockwell rear end with no options other than swapping r&p and I was told I'll blow up the overdirve untis that were used on older RV's.

Great ideas though,

Thank you Jaxom

Mikey D

Thank you Jitterman. I'm on the high side of that range right now. I took it up to 75 today and was at around 2600ish..

Almost sounds like I should leave well enough alone...

bigfoot

^^^^^^^^^^^^^^^^^^
This may be your best option.

But if you look around at some of the local Truck salvage yards you could find a used rear-end with a two speed that fits for about the same price (or less) as the R&P.

Jaxom

Years ago, 1987 I think, I had a Ford F700 with a 5 speed trans w/ a 2 speed rear axle. Didn't really care for the solenoid that did the switching. Liked it when it worked. Had I kept the truck longer I would have made something else, cable or air actuated. Axle itself worked flawlessly.
Good Luck.

12valve@heart

Before I spent money on regearing the rear end, I think I'd try it like it is and see how it pulled. I'm not sure that you will be able to pull grades at speed (especially 75-80) with your current combination but I'd wager that regearing to 3.42 would make pulling the hills worse.

mrclean

If you are running 41" tires, you will not want to regear to 3.42's. It would bring the cruise rpm down, but your vehicle will be ******* on hills. Much better choices would be the 2-speed rear others have mentioned, or a Gear Vendors overdrive unit.