Replace 4 speed with 5 Speed??

I have a 1986 ford F350 Dually pickup that I am reconstructing. The problem is no speed. It will reach 3000 rpm at 50 mph. I has a 4 speed manual transmission and i wanted to know if you all know if I can put a 5 speed in it?? Or if there is another solution to this problem??
Please help!!

I don't know that the problem lies in your tranny as much as it does in your rear end gear, with it being a dually it's more for pulling anyway so there's probably a low gear, the five speed would give you that extra running gear for the highway you would lose some pulling power though. I would suggest maybe putting a little bit higher of a gear and keeping the 4 speed for the pulling power, just a suggestion :smile:

The 4 speeds are GOOD trannys, the 5speeds are NOT. Change the rear gear to like a 3.55 instead of a 4.10 (or even a 3.73) and that will help a lot. If you don't pull a heavy trailer, you could even go a step lower to say 3.32.

Thank you. How is it that I can change the rear gear?

either buy a gear new or just buy the whole rear end from a junk yard or something if your not good at the whole mechanic thing I would suggest just to buy the whole thing it's easier unless you have the right tools and so on. or just take it to the local mechanic and let them do it. If you just buy a new gear it can be quite expinsive, a used one usally runs about $75-$150 depends on what.

Well the M5OD is not a good tranny, but the 5 speed ZF tranny is a tough tranny. The ZF's were in the 250 and up series. Any speed tranny is a strong tranny, nothing beats a NP 435.

The ZF is nowhere as good as the old 4 speeds (w/creeper 1st gears). They are not "HD" at all, despite Ford saying they are. They EAT input bearings, and are completely intolerant of abuse.

If I lower the rear end ratio to 3.55 for highway speeds, won't I have to modify the front axle differential also?

Find out what gear you have in the rear and the front, you have a small amount of play between them(not much) but you would be surprised just by changing the gear a small amount would help

You are "allowed" exactly o.01 of a difference F to R in gearing. You can have a 4.10 front w/ 4.11 rear, but NOT a 4.12 rear for example. More than that, and you'll put excessive wear on the tires and probabally break the chain in the transfer case, if not explode it altogether.

If you don't wanna change diff. gears, just throw on some bigger tires....it'll do the same thing.

Tires Are The Cheapest Route. They Make A Big Difference. Taller Will Give You Better Top Speed But Will Hinder Stop Light Acceleration.

i would change the tires a little bit, but a ZF 5 speed is a strong tranny from my expiriences. Another option would be an over-under drive unit.

i would change the tires a little bit, but a ZF 5 speed is a strong tranny from my expiriences. Another option would be an over-under drive unit.