K3500 Trans/gearing help

[up2itnow]

Hey Everyone, I finally got the square body I have wanted for years and I'm psyched. I bought it with the tranny (th400) pretty much dead and the original 454 very well used. I am planning on replacing both and I'd like to get it right the first time so your advice would be greatly appreciated.

Here's the deal. I have a 1984 K3500 crew cab with 6in lift and 35's. I was told that the diffs are 3.73 but I'm not sure. I do know that at highway speeds, the RPMs are pretty high. The plan is to put a mild build on the motor (300-400hp), get the TH400 rebuilt and mating it to a Gear Vendor under/overdrive to help the MPG. Does this sound like a good plan? If it turns out that I do have 3.73's should I change them? and to what?

The truck will be a driver and a towing rig for the travel trailer in the summer.

Thanks for your input.
Bill

 

[HotRodPC]

Welcome to GMSB Bill !!! You did almost all right for a new guy. You posted in the right category, and plenty of people here to help with all those questions, and you'll even get 50 different opinions, but you didn't post pics of your new rig. Unlike most forums, we're pic ****** here and this forum even supplies the space to post'em up. So when you get a chance, throw a few up. By the way, you've got the very truck that about half of us on this forum would love to have, especially if its still got the OEM Dana 60 up front and the NP205 Transfer Case as it should. So you of all newbies, WILL NOT get by without posting pics !!! Not on this forum.

Peronally, I think you got a good set up for what you want. Unless you just have money to burn, I'd leave it alone as is and just build the motor and trans you have. Now if you want to spend all that big money, which I really doubt you'd recoop in fuel savings in 10 years since Gear Vendor is a very expensive unit, plus driveshaft mods and all of the above, then I'd go with a 4.88 or 4.56 gear and the Gear Vendor unit. Add up gear changes, Gear Vendor and Mods to install the Gear Vendor, but please, be sitting down when you do it, and I hope your heart is in good shape when you see the cost.

Most of these Crew Cab K3500, oh and it's a GMC too, COOL !!! Most come with 4.10's. Some will disagree, but I like the 3.73's with your set up. You've got the Big Block for low end grunt to get it moving, you've got the Th400 which is very strong and can handle some strain and towing weight on it's own. And if in a bind where you need a bit lower gear to rolling or moving up a hill with a heavy load or a biggo boat up a steep launch ramp then you have the 4LO that just about doubles your 3.73 to a 7.46 ratio and if 7.46 won't pull it with a big block, nothing will. I really don't think you'd need an OD with 35 in tires.
With 3.73 gears, and 1 to 1 ratio on 35in tires puts you at 2500 rpm @ 70mph. Add 200 rpm for the loss of torque converter and not having a lock up, so 2700 rpm @ 70mph. 2400 or 2500 would be better, but for the size of the vehicle and being 4x4, I don't think that is bad at all. 80mph would be right at 3000 rpm or just under and that would be your ideal max cruising rpm for a long trip, but I've ran 3500 rpm before with 4.56's on 31's before, motor sreaming but begging for more. (Granted, that was a small block, not a big block). But, it's your truck and you're the one that has to love it. What I'd do first is build the trans and drive it a bit and see how you like those 3.73's. Then build the tired motor and see how you like the 3.73's. You know you're set on the 454/Th400, so nothing lost by getting them up to par then seeing what kind of power and mpg you have and if you don't like it. then make gear changes and/or Gear Vendor addition.

 

[HotRodPC]

Oh, and the other option I'd suggest if you're deadset on Overdrive, is just to do a 4L80-E conversion. Just about the strength of Th400 but with a .67 overdrive and lock up. With that, some 4.56 gears would be nice, but a pricey swap. I think I'd stick with the 3.73's. You can get the TH400 build pretty cheap for now, then keep your eyes open for a 4L80-E and a stand alone computer to operate it. The stand alone computer in OEM is in Diesel trucks only and only I think it's 91-93 year models. Someone would have to correct me on the year models cuz I'm not 100% certain about that. Hard to find though. Not only rare, they are grabbed up real fast by others doing the same thing.

 

[89Suburban]

Welcome aboard Bill.

 

[bucket]

Howdy!

I've got a crew cab K30 with 36's and 4.56 gears. It's also a mild 454 and a th400 and if you ask me, the combo is just about perfect. 3.73 gears aren't going to gain you any mileage, it is a big heavy truck with 35's after all. With OD, I would even think about 4.88 gears.

 

[HotRodPC]

He already has the 3.73's in it. That's what it has OEM. Why I said, just leave it alone. It's already tall enough for 35's, even down to 33's, but I don't think I'd go taller than 35's with 3.73's.

 

[up2itnow]

Thanks for the great info HotRod! I will definitely get some pics up tomorrow.

I have a little money to spend on this project but I really want to spend it wisely so I like your idea about replacing what I have and then seeing how I feel. Those Gear Vendor units are pricey for sure. I found one used for $1800 but new they're $3500...oouch. Plus gears, plus drive shafts, plus plus plus.

So as part of my purchase of this truck I also got an '85 C30 Crew Cab project too. It's been lowered and has airbags. The body is rust free and the mechanics are decent. I believe this truck has 4.10's so I could swap this rear end into my other rig to save a few bucks if I need lower gears. I also plan to strip out the AC and pollution equipment from this truck as well. Once I take this stuff I am going to off load it. I will throw a pic of this one up tomorrow too.

If you had to guess, would the 4L80-E swap be much lower cost than the GV unit? Would it hold up to the towing load? I don't want to end up with a glorified 700R4 that's going to grenade as soon as I put a load on it.

Thanks again for the advice!!!

Bill

Kind Regards,
Bill Wilson

 

[crazy4offroad]

4L80e is pretty much the same thing as a TH400 but with overdrive. It is no cupcake, it's a heavy-duty trans. You know I was thinking 3.73s with 35s on a truck that heavy may actually be hurting mpg if the cruising rpms are below the optimum power range. Do you have a tachometer in the truck? It would be nice to know what rpms you're turning at 70mph or whatever your usual cruise speed is.

EDIT just ran it through the gear ratio calculator and it looks like you're turning about 2600 rpm at 70 mph so it should be in the meat of the power range. As for the cost difference between gear vendor versus 4L80e I think it all depends on what kind of deal you can get on the 4L80e and the stuff to finish the install. You could end up way ahead of the game.

 

[Stroked]

Welcome to GMSB!

Wait you have TWO crewcabs?!? You lucky dog. Lol.

 

[up2itnow]

Welcome to GMSB!

Wait you have TWO crewcabs?!? You lucky dog. Lol.

Yep, I got two and I wish I could keep both but one project is enough so the 2wd will likely go soon.

Here are some pics of my 4x4

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

.

 

[up2itnow]

Here is two 2WD. Any takers?

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[up2itnow]

4L80e is pretty much the same thing as a TH400 but with overdrive. It is no cupcake, it's a heavy-duty trans. You know I was thinking 3.73s with 35s on a truck that heavy may actually be hurting mpg if the cruising rpms are below the optimum power range. Do you have a tachometer in the truck? It would be nice to know what rpms you're turning at 70mph or whatever your usual cruise speed is.

EDIT just ran it through the gear ratio calculator and it looks like you're turning about 2600 rpm at 70 mph so it should be in the meat of the power range. As for the cost difference between gear vendor versus 4L80e I think it all depends on what kind of deal you can get on the 4L80e and the stuff to finish the install. You could end up way ahead of the game.

I don't have a tach and the tranny only made it half way home before it quit, thank God for AAA. However, at highway speed it sounded like it was turning way more than 2600RPM. I am wondering if the previous owner was wrong about the 3.73's. I need to do some investigation to find out.

Bill

 

[DoubleDingo]

The best way to figure it out is to do the math.

RPM = mph x gear ratio x 336 divided by (Actual measured or design) Tire Diameter.

Hope that helps. I have set up a few vehicles this way and it works perfect every time. You get the desired rpm's for highway speeds and the tires and gears you want too.

 

[DoubleDingo]

I don't have a tach and the tranny only made it half way home before it quit, thank God for AAA. However, at highway speed it sounded like it was turning way more than 2600RPM. I am wondering if the previous owner was wrong about the 3.73's. I need to do some investigation to find out.

Bill

You will need a Tachometer to complete the formula, however.

In my Jeep Cherokee I have 3:73 gears by design from the formula. Tires are 29.5 inches diameter. At 70 mph the motor is turning around 3,000 rpm. In OD it turns about 2,200 rpm's. I used that formula to get the engine turning close to what the factory had it with the tiny tires on it.

 

[DoubleDingo]

And welcome to the board! At work so I am doing this quickly in hopes I don't get caught...lol...