Add-on overdrive options for a '90 F250 2wd 7.3 idi with ZF5?

[scsmith42]

I have a '90 F250 2wd 7.3 idi project truck that I'm bringing back to life (been sitting in a barn for several years with a bad oil cooler), and ultimately intend to mate it's front half to the rear half of an '82 F350 dually with flatbed dump that I have. Truck will be used here on the farm but will see occasional highway use. The F350 has 4:11's in the back. Not sure about the F250, but it's tacking around 2,400 rpm at 65mph so probably 3:73 or 3:93.

The 250 is in nice condition and (now) runs well.

What are the best options for adding a manual overdrive (auxiliary transmission) behind a ZF5? I've got to redo the driveshaft anyway when I do the swap so adding in an auxiliary should not be a problem. I knew a guy that installed an old Brown Lipe auxiliary transmission behind a late 70's F350 back around 1980, but those transmissions are getting hard to find.

Seems like most of the auxiliary transmissions available are for Class 7 or 8 trucks are way overkill for a F250 / F350 sized truck with a 7.3 idi.

Advice welcome. Thx.

Scott

 

[FrozenMerc]

That is a tough row to hoe these days.

My '85 F350 with 3.73:1 rear gear and a ZF5 turns 2500 at 75 mph. I pull enough with the truck that it is the perfect ratio with the big enclosed hanging off the bumper. If I didn't pull as much as I do, I would have gone to a 3.55:1 ratio, or even a 3.31:1 (yes they are available for Dana 80's) in the rear. A don't think my motor (7.3, 90 cc CDD Pump, Turbo, and Injectors, Oregon Cam, Banks exhaust) would be happy at all with a double overdrive. You would have to build and tune a motor to produce a whole bunch of torque around 1600 rpm to make a double overdrive work. It can be done, just not easily with a short stroke V8. Much easier with a long stroke DT466 or similar.

There isn't much available for medium duty or light duty sized stuff other than the old Brownie's which as you stated are getting few and far between. Auxiliary boxes are going the way of the dinosaur and the only stuff that is readily available is large class 7 and 8 stuff.

I think GearVendors still sells an overdrive setup for the ZF5, but it ain't cheap. I would stab a 3.31 gear set into the rear and call it day if you want to run those kinds of ratios.

 

[KansasIDI]

I have a Spicer 5831 with the 0.73 overdrive. I was gonna install it in my service truck about a year ago, but never got around to it. Now I’m working on a different project, I’m no longer looking to install that auxiliary box. I’d be willing to sell it, could ship via commercial address.

 

[ROCK HARVEY]

Are you sure your tach and speedo are reading right? I have an 89 with a zf-5, 4.10 gears and 35” tires, and 2400 rpm in 5th gear is over 80mph.
2400 rpm in 4th is closer to 65mph though.

Any chance a previous owner put in 5.38 gears?

 

[IDIBRONCO]

From what I remember seeing, the 2500rpm at 65 in overdrive is about right for 4.10 gears and 235/85R16 tires. A lot of the trucks that I test drove turned 2000-2100 RPM at 60 in overdrive.

 

[KansasIDI]

Y’all should try an IDI with 5.13 gears

 

[asmith]

I have a '90 F250 2wd 7.3 idi project truck that I'm bringing back to life (been sitting in a barn for several years with a bad oil cooler), and ultimately intend to mate it's front half to the rear half of an '82 F350 dually with flatbed dump that I have. Truck will be used here on the farm but will see occasional highway use. The F350 has 4:11's in the back. Not sure about the F250, but it's tacking around 2,400 rpm at 65mph so probably 3:73 or 3:93.

The 250 is in nice condition and (now) runs well.

What are the best options for adding a manual overdrive (auxiliary transmission) behind a ZF5? I've got to redo the driveshaft anyway when I do the swap so adding in an auxiliary should not be a problem. I knew a guy that installed an old Brown Lipe auxiliary transmission behind a late 70's F350 back around 1980, but those transmissions are getting hard to find.

Seems like most of the auxiliary transmissions available are for Class 7 or 8 trucks are way overkill for a F250 / F350 sized truck with a 7.3 idi.

Advice welcome. Thx.

Scott

Unless someone swapped in the gears these trucks did not come with 3.73 or 3.93 gears. Only 4.10 or 3.55. What size tires are you running? that has an affect on the rpms you see.

 

[scsmith42]

From what I remember seeing, the 2500rpm at 65 in overdrive is about right for 4.10 gears and 235/85R16 tires. A lot of the trucks that I test drove turned 2000-2100 RPM at 60 in overdrive.

I think that you hit the nail on the head. That’s the current size tires that I’m running.

 

[scsmith42]

Unless someone swapped in the gears these trucks did not come with 3.73 or 3.93 gears. Only 4.10 or 3.55. What size tires are you running? that has an affect on the rpms you see.

235/85r16. I’m thinking it has 4:10’s based on the feedback in this thread.

 

[scsmith42]

Y’all should try an IDI with 5.13 gears

Wow, that must be a screamer!

I used to have an 08 F450 with a 6.4 and 4:88’s. It was a great towing truck but the heads kept cracking.

 

[u2slow]

With the IDI/ZF5, 3.54 is nice for a lighter truck that's an everyday driver and highway travel. Low range is still there for tugging things around. 4.10 would be my choice for a heavier truck or needs to haul more regularly.

No need for an aux overdrive, IMHO. Those rpms are nothing to sweat about on a V8 diesel.

 

[Indadesert]

I’d say hunk I might just slow down. At 55-60 empty I get well into the 20s mpg. 33” with 3.55s. E4od 7.3 with a turbo. I’m going 4.56 gears in the spring.