Need help deciding on new transmission

[Dstrm1990]

So, I have a 71 SWB K10 with a 350sb, turbo 350, np205, 33in tires with 2 in lift, and 4:10 gears front and back. Rpms are crazy at highway speeds. I am trying to make this truck capable of driving long distances at 70 or so mph. Question is whether to just do a tranny swap or do both tranny and gears. Also, what scenario would y’all go with?

[jocko]

3 options:
1) lower numerical (higher) gearing - such as a 3.73.
2) 700R4 or 2004R o/d trans
3) Gear Vendors over/under drive.

$ permitting, I like option 3 as you can then keep the entire driveline bone stock (except rear driveshaft) (Gear Vendors makes a 205 kit and it used only in 2WD, of course).

Also keep in mind that an O/D trans may necessitate moving the engine fwd a couple inches, but many have done this, the holes should already be there, lots of threads here about that swap.

Gear change AND an O/D trans will get quite pricey on a 4x4... will likely exceed the cost of a GV unit - or at least be about a break-even best case if you get a trans for cheap. And that's just for parts. A sturdy O/D trans will run you $1800-3000, depending on where you go for it - and 2 sets of gears (plus install if you don't do it yourself) will add up quickly. GV O/U will be about $3100-ish, and you'll need to shorten or make a new driveshaft (I'd do that latter and preserve reversibility)

First row shows highway rpm for current and other trans options
Second row shows highway rpm for current everything + GV over/under drive (only change)
Third row shows highway rpm for new 3.73 gears front and back (only change) (in other words - 3.73 swap alone will not make you much happier).

3.73 swap is a 10% reduction in rpm at highway speeds, but you decrease 1st gear torque multiplication. (not worth it)
700R4 is a 30% reduction in highway rpm, and you increase 1st gear torque multiplication.
2004R is a 33% reduction in highway rpm, and you increase 1st gear torque multiplication (a little bit).
GV is a 22% reduction in highway rpm with no change in 1st gear torque multiplication - but you also get "an overdrive in each gear" making your TH350 a 6-speed. Many just use it as a basic overdrive, i.e. only as an overdrive 4th gear, if you will.

When my time comes for this, and it will, I have the exact same setup as you, except in a K20 with slightly smaller diameter tires, my plan is to go the GV route.

[harpo231]

3:73 and an OD ....

[Dstrm1990]

Jocks, Thank you soooooo much! I am having a bit of a time finding a good shop to do the work correctly in the ft. Worth area. I see you live near me in Keller. Do you have any suggestions? Regardless, I want you to know your input will be taken very seriously.

[Dstrm1990]

I put Jocko, and see what I get.

[Dstrm1990]

I put Jocko, and see what I get.

[Bigdav160]

200-4R never came in 4WD configuration and will not bolt to your NP205

[Mike C]

How important is price? If you are as cheap as I am I’d be looking to swap axles with someone. 3.42 would be ideal but not sure that I’ve ever seen a matched set. 3.73 would reduce your rpm by 10% which is significant. 3.07 are very common and they will lower rpm 25%. But mileage may not improve because the 33’s May take motor below peak operating efficiency.

[jocko]

Mike’s Off Road in Haltom City comes to mind - not far from you. I’ve been in there but never used them personally, so can’t attest to quality, was just looking for some parts. They do tend to always have a bunch of trucks seemingly lined up to get worked on - so they are either good or slow. Advanced Adapters is also a good company to keep in your roll-o-dex for adapting transmissions and transfer cases. Good point above by bigdav about 2004R, but AA “may” make an adapter. Keep in mind if you go shopping for a 700R4, you’re looking for a “K-Case” trans. Bowtie Overdrives carries some nice K-Case transmissions and they also sell all the TV cable stuff as well. Good expert customer service too. They’re in SoCal but ship everywhere. There is a local guy on Dallas craigslist as well that builds K-case 700R4s as well. I spoke to him once, seemed very easy to work with. Sorry don’t have his info handy.

[Rufas]

In 2011 I installed a BowTie Overdrive 700R Level 3 trans in my 67 Cutlass with a 468 BBC. The 468 dyno'd at 518HP and 586 torque. I've not had one problem with the trans.

I noticed BTO offers the 700R4 K-Case Level 3 for 4x4 applications.