Now don't get mad from title kuz I know zf5 has overdrive but I would like to know something about adding an external overdrive. My truck is 1991 f250 7.3 idi banks turbo with 4.11 diff gears and 4x4. I think that's about all but doing highway speeds in od the rpm are to high for me and would like to know if I could do something about it without changing the diff. I was thinking of putting lift and 35" tires but that won't affect rpm much with tire size difference. Open to all suggestions

 

An external overdrive unit would have to go between the tranny and transfer case, in order for the 4x4 to work properly. At the very least, that means custom mounting, custom driveshafts, and a very long linkage for the 4x4 shifter.

Stock tires are aprx. 31" diameter, so 35s would drop your RPMs by aprx. 4/31, or about 13%. You wouldn't actually see it unless you recalibrated your speedometer (I think that means a different speedo gear, in your generation of truck). Or you'd just have to know that, for example, when your speedometer says 65, you're actually going about 73 MPH.

 

What RPM are you currently running at, say, 70 mph? Why do you feel like your RPM are too high?

 

I'm not great with mph but at about 110 I'm at 4250 rpm, you think with tire change it would make enough of a difference?

 

It's about 4250 at 110 kmh the speedometer is attached to the tranny and I can figure out how much I will change the gear after I do tires but do you think the 35 tire will make enough of a change

Srry bought the repeat still figuring out this post thing

 

Another thing, I'm not to worried about keeping the external overdrive with 4x4 I'll just stay in the tranny overdrive but could I add it to the transfer case for just the rear wheels and change the length of the rear drive Shaft

 

OK clearly your have something else going on:

110 km/h is roughly 68 mph for us US folks.

Your truck should be revving less than 2500 rpm.

I'd be checking out the tachometer. Your engine will not spin that fast unless it's been modified.

 

It's helpful to put your location in your profile; if I'd seen a GWN location I would've translated. Anyway, 110 km/h is about 65 MPH, and you should be doing only a little over HALF that RPM. I don't have a tach in my IDI, but AFAIK our trucks redline at about 3000-3500. I didn't even think the diesel tachs went up that high.

 

OK well I'll look into it but put that aside is it possible to use the external overdrive for the rear wheels?

 

I might be thinking in terms of gas engine since I don't have the truck near me at the moment I looked it up and it looks more like 2750 still way to high srry bout that, was a vary stupid mistake.

 

Why do you feel that is too high? These diesels (and yes, uh, that would make a BIG difference) can run at the governor all day long with no adverse effects. Only issue might be fuel consumption.

With all that said, 35" tires will drop your rpm 300-400, depending on speed.

However, the best way would be to re-gear. My truck, set up similar to yours, has 3.55 gears, and runs right around 2,000 rpm at 70 MPH.

Yes, it would be possible to use the overdrive for the rear wheels only. However, that would NOT be the right way..........and you would most likely have to mount it behind the transfer case, with a stub shaft between the tcase and the OD unit. By the time you do all that, you may as well re-gear.

JM2CW

 

I think it's to high because the highway speeds go up to 130 here and that bit of difference affects the rpm to much for my liking, driving 110 is as fast as I'm comfortable with at the moment but the 400-500 rpm drop will help

 

just know that changing gears or tires will negatively affect power. less mechanical advantage.

 

I'm aware of power loss the turbo will be tuned for 10psi or so but I'm not a power junky so that's fine with me

 

You might be able to find so.eone with 3.55's that would trade for your 4.10's. I guess ypu can get a gearvendor to go behind you transfer case but that would cost what... $3000 new or $1000 used, if you can find one.