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Old 09-28-2017, 07:11 PM   #1
bcoke
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Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Hi! I'm new to the forum and have only had my blazer for 6 months, so you'll have to pardon the ignorance. I'm looking for some advice on a problem I'm having: how to get my 72 Blazer on the highway.

It seems like the last owner wanted it to be a rock crawler, but I'm looking for it to be more of a daily driver. Right now she screams at 55. My goal is to be able to eventually take her on a road trip and cruise the highway at 70.

Here are the current specs:
TPI 5.7L 350 engine
4 wheel drive
Turbo 350 automatic trans
NP205 Transfer case
4.83 gear ratios
1 ton axles
35in tires

I don't need the towing capacity of the 4.83s (at least not right now), but if possible I'd love to keep the torque while increasing the max speed. I know that that might be asking for too much, though, and just might not be practical.

I prefer a manual trans, so I'd like to eventually swap out the automatic for a manual, but I can hold off on that if there'd be a better way to get the truck up to highway cruising speeds right now.

It looks like re-gearing the diffs (have to do front and back since its 4WD) will be around a $2,000 job. That's creeping toward some tranny swap prices I've seen, and it seems like it'd be a shame to knock down a beast like this to something like a 3.55.

So my questions are...
1. Would a swap to a manual tranny like an NV3500 give me what I'm looking for in terms of top speed while keeping that low-end torque? Or would I have to regear the diffs even with the tranny swap?
2. Would adding an extra gear to the current trans setup give me the required speed at a price that's more reasonable than a tranny swap or a diff regear?

Any advice or ideas would be greatly appreciated. I'm open to everything. Thanks in advance for the help!
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Old 09-28-2017, 08:28 PM   #2
franken
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

The NV3500 is considered a bit weak. You might consider the NV4500 as well. You'd need a 4x4 version and the NP205 you have won't bolt up unless its a late one w/ round input pattern The input is also 27 spline and that won't work with the NV4500. I can't speak to the NV3500. You should be able to get an adaptor to bolt to the NV4500.

Of course you need a bellhousing and clutch pedal, linkage and so on.

I think a 700R4 might be the easiest swap, but again, mating it to the TC could be an issue.

Try checking out Advance Adapters conversion guide.

The following search has a K10 swap TH350 to 700R4 link. You can do other searches as well.
https://www.google.com/search?q=k10+....0.269zFrHoYn8
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Old 09-29-2017, 12:04 AM   #3
tjsblazer
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

I second the NV4500 if you want a manual with an overdrive gear but it will be more than $2000.
You don't want to go through the installation swap without having a fresh tranny and transfer and as mentioned you need a newer (I think 1989+) if I remember correct round pattern NP205 and machine the tailstock on the tranny a bit or you need an advanced adapter which are $$$ .
New clutch.
Probably hydraulic master and slave.
Custom transfer case shifter linkage.
It creates a slightly longer drive line so you need to adjust length of both driveshafts or move the engine ahead about 1.875"
Also need a custom tranny mount made for the crossmember.
Probably some things I'm missing while I type this quick but you can easily see how this adds up to quite a bit more that $2000.

Guess how I know. You can take a look at my build thread in my signature.

I can't comment on a tranny change to auto with overdrive.

It is impossible to maintain the same bottom end pull and lower highway speed rpm without a tranny change, however, in all honesty only a gearing change is probably the easiest route if budget is any concern.

I'm not sure what gear ratios you can get for your axles, however, I think you would be looking at either 3.73 or 3.55 considering your set up without any other changes. Kind of a middle ground gear with 35" rubber and your drive line. At 65 mph it would be around either 2460 rpm or 2340 rpm. So not a huge difference. I guess it depends how torquey that engine is on the bottom end which one would be better.

I love the site linked below. Use the drop downs to enter your tranny, transfer, gears, tire size and then you can play with the outputs and compare side to side.
http://www.grimmjeeper.com/gears.html
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Last edited by tjsblazer; 09-29-2017 at 05:48 AM.
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Old 09-29-2017, 12:06 AM   #4
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

I'd go 700r4 all day buddy. I would imagine that you could find somebody to build it pretty stout to go with the rest of your heavy duty running gear.

I'm running a th350, 205 transfer case, 4:11's and 35's... but mine will be a weekend cruiser, with an occasional drive to the coast. If I was looking to drive it on the hwy a lot then I would have gone with a 700r4.

I agree with the previous post, Adavanced Adapters sells a great product. Unfortunately though, buying a friggin adapter close to 300 bucks is like buying a washing machine. You definitely know you spent the money, but you just can't get excited about it. Lol.
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Old 09-29-2017, 02:20 AM   #5
bcoke
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Wow thanks a ton for all the info everybody!

I think I agree on the 4500 vs 3500 deal - I've been researching it for a few weeks and was originally set on the 4500. Then I met a guy who offered to rebuild me a 3500 (he's done it a bunch and swears by it), and I heard the 3500s drive a little easier while the 4500s drive like dump trucks. But I think in the end I'd rather go for the tougher 4500 because I would be pissed if I had a 3500 rebuilt and still blew it out.

@tjsblazer: Thanks for the rundown, and I'll definitely take a look at your build thread! And yeah I'm getting the feeling that it would be cheaper and less time to gear down...I just hate to spend $2000 giving up torque instead of putting that towards my love of manual. But budget is definitely a concern...I'll have to think that one over some more.

@spf79: I actually only recently heard about the 700r4. I know it's one less gear than the 4500 but seems to stack up similarly (and actually even a little better) with my parameters to the 4500 in terms of highway speeds and RPMs. Any advice on differentiators between the 700r4 and the 4500?
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Old 09-29-2017, 03:29 AM   #6
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

I don't have much for comparisons...

I like the idea of a heavy duty manual... but that is a pretty big swap to do, and it all adds up dollar wise.

The th350 to 700r4 should be a bit easier, you will need this adapter/spacer.
http://www.advanceadapters.com/produ...with-gm-700r4/

Do you have anything that supports your 205 transfer case other that the cross memeber? If not, you may want to get something like a torque rod or an after market brace for the back.of the 205. See the pic of the torque rod.

You should be able to use your current th350/205 adapter... double check me on that, Google says yes... and everything you read on the internet is true. Lol
You will also have to move the transfer case back a little bit, more than a couple inches, less than 5 I think. This creates driveline modifications... or you can try and move your motor mounts forward and split the difference.

I have always heard that 700r4's are a crap tranny, not being known for liking oversize tires or towing. On the flip side, I have known a few people that have had them built with abuse in mind and they have been solid, no issues. That is what sold me on them. Personally I didnt want to spend the money on the 700r4 build when the th350 fit my needs better. (Note, run a big tranny cooler)

For what you are looking for I would go 700r4., Taking a quick peak on line, it looks like it will be your cheapest option.. depending of course on how crazy you go with your transmission rebuild. I saw that you could have a stout 700r4 for under 1,500... looks like 1,100 for a stock rebuild on line, shipped to your door with a 24 month warranty. .

I hope this was helpful, and Good luck!!!
Let us know what you decide!
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Old 10-01-2017, 07:29 PM   #7
stangtjk
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

$2000 seems really steep for changing out gears. I'd shop around for a more reasonable shop. For that kind of money you could just buy a Dana 60 and 14 bolt with the gearing you want and swap them in there.

I just had to put gears in my 10 bolt front because the person that swapped the axles didn't get ones with the same gear ratio. Cost me about $300 to do it myself and that's including replacing all the seals and bearings and buying a case spreader to set the carrier bearing preload.
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Old 10-01-2017, 08:34 PM   #8
hemi43
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

I just put gears in the rear of my Blazer. Gears were $279, bearings around $80 plus a bit of gear lube. Total around $400 per diff. Do it yourself, it's not that difficult.
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Old 10-04-2017, 02:20 PM   #9
cleszkie
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Throwing in an overdrive transmission is not going to overcome your 4.83gears. If you want the benefit of an overdrive, you need to have some more reasonable gearing in the axles. Something more in the range of 4.10 with 35" tires.
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Old 10-04-2017, 09:23 PM   #10
franken
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

BTW, Dodge and GM 4500s are a bit different and different years are a bit different. My choice is a 96 or newer GM trans since the 1st gear is less steep (5.6:1 if I recall) and has a synchro.
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Old 10-05-2017, 12:56 AM   #11
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

You could run a gear vendors overdrive unit. Cost about $3k though.

Don't remember what the gear ratio would be, but a 0.75 overdrive would drop your rpm by 550-600 at 55 mph.
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Old 10-05-2017, 01:13 PM   #12
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

Quote:
Originally Posted by cleszkie View Post
Throwing in an overdrive transmission is not going to overcome your 4.83gears. If you want the benefit of an overdrive, you need to have some more reasonable gearing in the axles. Something more in the range of 4.10 with 35" tires.
I have 4:10's with my 33"s and OD transmission. I definitely would not want a stiffer gear. I think he'll be fine running 35"s with an OD transmission IMHO. A quick calculation puts him around 2000 RPM @ 60MPH which is perfect.
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Old 10-05-2017, 09:38 PM   #13
Mike C
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

I have 4.10 in my Jimmy with SM465 and 35" tires. This is a great combo, especially if you are good with 65 mph cruise speed. If you want to run the interstate at 75-80 then may be too much gear. But those speeds aren't what these trucks like best anyhow.
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Old 10-06-2017, 12:52 AM   #14
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Re: Advice Needed on Diff Re-Gear/Tranny Swap Dilemma

I run 4.10 gears with my SM465/NP205 combo, 35" tires, and a Gear Vendors unit off the back of the NP205. The 4.10 gears and 35" tires without an overdrive is a very drivable combination, both on and off highway. But here in SoCal, you need to be able to cruise at 75-80 mph to keep up with traffic!
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