LM: Driveshaft Critical Speed

Me personally, I'd put 5th where 4th is now or a LITTLE higher and let 6th be whatever it will be, which is probably still usable but just not in every single scenario. 

The real question is whether making 6th gear line up with current 4th just makes driving the thing tedious. I was just back in my buddies turbo miata (pressed some wheel bearings and did a motor mount) and even with the 3.63s the 6 speed is still annoyingly 'busy'. 

Vigo (Forum Supporter) said:

Me personally, I'd put 5th where 4th is now or a LITTLE higher and let 6th be whatever it will be, which is probably still usable but just not in every single scenario. 

The real question is whether making 6th gear line up with current 4th just makes driving the thing tedious. I was just back in my buddies turbo miata (pressed some wheel bearings and did a motor mount) and even with the 3.63s the 6 speed is still annoyingly 'busy'.

Six is one too many gears. 

Streetwiseguy said:

Vigo (Forum Supporter) said:

Me personally, I'd put 5th where 4th is now or a LITTLE higher and let 6th be whatever it will be, which is probably still usable but just not in every single scenario. 

The real question is whether making 6th gear line up with current 4th just makes driving the thing tedious. I was just back in my buddies turbo miata (pressed some wheel bearings and did a motor mount) and even with the 3.63s the 6 speed is still annoyingly 'busy'.

Six is one too many gears. 

When driving on the street I almost never use every gear with a 6 speed. 

There's no reason not to have a nice relaxed 6th gear, it helps Corvettes get 30+ MPG on the freeway. The 4L60 overdrive is a lot like a T56 5th gear.

I would either stick with the 3.42s or get a set of 3.73s. 

No one has written this yet, so I will.  Just put it together with the T56, 4.56's and stock driveshaft and see what happens. I'm with the driveshaft shop: it'll be fine.  And if it isn't, just have another driveshaft made, which is what you appear to be thinking about anyway given the question you asked. 

Also, the later GMT 400 foam filled aluminum driveshaft (I have one in my '96 K1500) probably damps vibrations better than a steel one and probably has a higher resonant frequency because of the larger diameter and thicker tubing. 

Streetwiseguy said:

Vigo (Forum Supporter) said:

Me personally, I'd put 5th where 4th is now or a LITTLE higher and let 6th be whatever it will be, which is probably still usable but just not in every single scenario. 

The real question is whether making 6th gear line up with current 4th just makes driving the thing tedious. I was just back in my buddies turbo miata (pressed some wheel bearings and did a motor mount) and even with the 3.63s the 6 speed is still annoyingly 'busy'.

Six is one too many gears. 

Agreed.

Streetwiseguy said:

I have an NV3500 in my 4.8 Silverado, 3.42 gears, and I have never once worried about first gear being too tall towing a trailer.

I have a close ratio NV3500 and 3.42's in my 5.7 K1500. First gear is great when it's empty, but getting a 6K lb trailer moving pretty much sucks.  Those gears behind a 4.8 would be a bunch worse. I'm looking at 4.10's and the wide ratio NV3550.

I think this is the first time I've heard that 6 is too many. All my manual cars have been 5 speeds, and while I've driven cars with 6 speeds, I never felt like it was excessive. That may not be something I'd notice without daily driving the car for a bit though. Very interesting.

The bikes are all 6 speeds except for one, and I'm always looking for another gear one the 5 speed bike. I also added a 6th gear to a bike that didn't originally have one with no regrets.

NV3500s are very thin on the ground around here, especially 2WD drive ones. My T56 is known good; I drove the car it came out of a few times. Most NV3500s would be unknown and it's hard to tell if they have the common intermediate shaft bearing wear/failure issue when it's out of the truck.

I'll keep an eye out, but will proceed with the T56 if nothing pops up in time. My original plan was driveshaft, transmission, rear end, but now I might do the transmission first to make sure the transmission is actually as unlivable as I expect it to be with the stock gears. Transmission, then driveshaft, then rear end if needed. The feedback on towing with the NV3500 makes me think I will absolutely need to regear with the T56 though.

I know it's easy to get lost in overthinking the numbers, but I have the shift maps for the 4L60 from EFILive. That lets me compare to the automatic while taking the torque converter into account, at least at WOT. It does illustrate how tight the gearing is on the T56.

4L60: 1>2 at 36 mph, 2>3 at 70 mph (5200-5300 rpm on the tach)

T56 + 4.56: 1>2 at 39 mph, 2>3 at 58 mph, 3>4 at 80 mph (5500 rpm shift)

NV3500 + 3.42: 1>2 at 34 mph, 2>3 at 59 mph, 3>4 at 99 mph (5500 rpm shift)

Just for giggles, here is the T56 + 4.56 vs. NV3500 + 3.42:

And NV3500 vs. 4L60, both with 3.42s. 3rd and 4th in the automatic are the same as 4th and 5th in the manual.

I think it just depends on how useful the 'spread' is. You can always skip gears, but that's kinda the same thing as thinking 6 is too many anyway.  I'm not saying your idea is bad. It'll still be awesome.