OK, i have driven my new truck to work now for three days, and have found one problem. I am only running 65@ 2200 rpm, 70 is 2500rpm. this does not make for good fuel economy. 4.10 rear. I would love to put a 6 speed in, but i was told today that it won't clear the floor pan, and ZF won't honor the warranty installed in a pre 99 truck. I am out for longevity, and i want to be able to cruise at 70 running about 2100 rpm's, I guess my 99 f350 spoiled me. So what is my best plan? should I keep the 4 speed auto and add an aux overdrive, or swap out to a five speed manual and add the aux overdrive? We have a 94 shop truck 5 speed where i work and I think it turns about the same rpms as i am right now. That swap would be the easiest, since I would not have to modify the floor pan, I prefer the manual transmission anyhow, and I don't want to fight a loosing battle trying to keep an auto in it. So when the transmission in it now goes, should i buy one of the performance built rebuilds, or plan on swapping out to the 5 speed and aux overdrive? i am keeping this truck for a long time, only has 54,300 miles on it right now, and looks new, so I can justify the 5 speed swap if it will make a difference, I haven't had a chance to tow with the auto yet, but I think the manual will always pull better. Any thoughts anyone?

 

The overdrive ratio in the ZF-5 and ZF-6 are nearly identical. What has spoiled you about your 99 is most likely the 3.73 axle gears. You could do the aux. o/d behind the five speed, but it would probably be more cost effective to put taller (numerically lower) gearsets in the differentials.

 

Over at dieselstop on the 7.3 forum I found a post from a guy looking for a particular rear axle. He intends to install 3.08 gears. I am not knowegable in this area but he stated that it would match the power curve of the diesel much better. Also a guy who has installed 3.08s already. Pulls a small excavator with no problem and claims 24 mpg. It is expensive but might be worth a look. Also said that the rear end swap was more effencient than an overdrive.

 

I don't know why one would be more effiecient than the other but one thing to consider is if the 3.08's aren't exactly what you need (or whichever gearset you choose.) what then? More gears. With the OD unit, either drop out of OD or downshift one.
When I regeared my early Bronco to 4.56's it cost $145 for the rear and about $300 for the front and that included a new carrier. But I made the gear swap myself.

 

I really need to keep the 4.10s that I have now, I tow horses, and get off road, and on a few 12% grades, also, my 99 did have 4.10's also, I spec'd it out and ordered it that way. Any one know if the zf 6 speed will fit? I kind of think the guy at the trans shop really didn't want to mess with it, and just gave me a dead end answer.

 

Well I am not sure about the whole six speed thing but I have an offer for you if you decide to go with the five speed. I have one in my truck right now and I want to switch to an auto but I am not able to find a donor one that has all the guages and everything that is required to swap it over to a factory style look. So if you are intersted send me an email and maybe we can work somthing out with some sort of a swap????

 

IMHO I thought an auto was better for pulling anyway ... can't you put in an auxilliary overdrive unit in the back of the auto to give you three more gear after your overdrive?

 

Still another option.....
I've got 4.10 gears and 35" tires, so I'm going 69mph at 2000rpms. And, at that speed, I get 19.5mpg. Putting on taller tires is a little less complicated than the other options.
Just my $.02.

 

If it were only a single rear wheel, the taller tires would be the way to go, hard to do with a dually. If it tows OK, with the auto, I am just leary of all the problems people have had with them, I can just add one of the overdrive units for $800, I have always liked the control of a manual better than the auto, but hopefully, this one will be able to change my mind. First I have to get the gooseneck hitch installed as well as a trans cooler

 

Where can you get an overdrive unit for $800??

I am looking for one to go behind my 94.5 4X4 ZF-5 spd..

I would really like to get one!!