Looking for opinions on gears/tranny

[JOJABOY]

Currently TH400 with 4.10 in 3/4 ton. Engine is screaming on the highway. No tach, but I'm guessing 5k just to hold it at 60mph. So I'm looking for opinions on whether to keep TH400 and swap rearends or keep 4.10's and start the hunt for a 700r4?

[jocko]

Surely not 5k! Ha. I hope not anyway! But, I feel your pain. I have a 71 C20 with 4.10s also... only difference is it's a TH350, but for high gear, they are the same.

If you happen to have a C20 big block truck lying around for parts, the 3.54 rear end would be a nice swap, but you will lose a good amount of first gear grunt. An o/d trans swap is a good option, and not too hard to do.

See the numbers below to see where you are now and what some of the changes might bring you as far as reduced highway cruising rpms (I plan to swap in a Bowtie Overdrives 2004R at some point):

[Already Gone]

Originally Posted by jocko

Surely not 5k! Ha. I hope not anyway! But, I feel your pain. I have a 71 C20 with 4.10s also... only difference is it's a TH350, but for high gear, they are the same.

If you happen to have a C20 big block truck lying around for parts, the 3.54 rear end would be a nice swap, but you will lose a good amount of first gear grunt. An o/d trans swap is a good option, and not too hard to do.

See the numbers below to see where you are now and what some of the changes might bring you as far as reduced highway cruising rpms (I plan to swap in a Bowtie Overdrives 2004R at some point):

Mark, what height tire did you use for these calculations?
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[leddzepp]

What kind of budget are you working with?

[jocko]

Originally Posted by Already Gone

Mark, what height tire did you use for these calculations?
Posted via Mobile Device

Oops - meant to include that - it was a 31.7" dia tire, roughly a 265-75-16

[JOJABOY]

I would like to keep it under $400 if possible. I haven't visited the local pick-a-part just yet. Still thinking about what I would rather do.

[JOJABOY]

I'm leaning towards 700r4. Just not sure what all is involved. I know I will need to do something with the drive shaft.....shorten or swap or something. Plus, wiring for kickdown? Not too familiar. I know vacuum accuator upshift TH400 and carb mounted kickdown down shifts.

[leddzepp]

A 700 tranny swap would be ideal, but I think you will have a difficult time doing it for under $400. A rear end swap to a 3.54 is very possible for under $400. That is the standard gear used, and the rear ends are plentiful here at the pick n pulls. You might also get lucky and find one that has posi. The last time I priced a drum-to-drum rear end it was $150 I believe. I had a 3.54 in my C/20 with a TH400 and was happy with the way it drove on the street, as well as the freeway. I thought it was a good balance.

[JOJABOY]

My TH400 shifts fantastic. It just seems to be missing something at the end. LOL I need to do brakes and was thinking that I might just go look for a 3.54. Then down the road decide on trans. no one said I had to get rid of the current rear end. I do have storage space.

[jocko]

agree w/scott - since budget is tight, this is indeed your best approach (most bag for the buck for now). Is your truck leaf or coil spring rear? Might focus on finding same type in the yards for ease of installation.

[JOJABOY]

leaf springs. was thinking about getting the 2 inch drop shackles to level it out.....and i need to do some brake work. going to install a booster to the drum/drum system. debating swapping out the rear end all at the same time. after lots of thought (as the wife watches the real housewives of who cares)....i think i will do the swap, but keep the 4.10 rear end and maybe look for a 4 speed tranny down the road some day.

[leddzepp]

On a side note, 71-72 C/20's are the most abundant here, and you can also get a disk brake swap for the front at the same time, if you are interested in doing that.

[JOJABOY]

Out of curiousity......what all is involved in a transmission swap? Other than the shift indicator on the column. Does 700r4 have the same speedo connection as TH400? Can I purchase a different drive shaft or would I have to modify the one I currently have? What about wiring and up/down shifting?