I am in the market for a good Differential and just cannot make up my mind. I see multiple good/bad things about each unit and wanted to here from some first hand experience.
It is for my truck, info in sig. I am looking for a diff that will allow me to do some 3,500-4,000 RPM 2-step launches but I will only be doing this maybe 10 times a year. I do daily drive this truck for now but eventually it will be just a fun street/strip truck. I am looking at 4.30 gears to go with it.
My rear end is a 8.5" 10 bolt

 

i believe Motive is the only one who makes 4.30s for GM trucks (or the only ones i found anyway). i've had mine in for almost a week now & like them so far. granted, i've only got about 120mi on them now & am still taking it easy during the break-in, but i they've got a noticeable bit more pick-up than the 3.73s.

now, i ain't racing the truck no more... that's what the car's for. and i've got heavy D-range 33s on my 4x4 with a new TrueTrac, but my 5-speed is geared lower than your T-56, so the math almost averages out to be a similar feel if you're running a 29" tire. i think the 4.30s would do as a pretty good compromise between launching effort & the cruising around you said you do most of the time... maybe 4.56s.

 

Awesome, thanks for the info. Yeah Motive is the only company I found with the 4:30's as well. I have been hearing about the TrueTrac so that is looking more and more like the one I am going with.

 

i spent a couple years looking into them & finally got around to getting one in. i'm still breaking everything in, so i can't comment on how they hold up to actual use yet.

 

The truetracs seem to be taking the abuse from alot of trucks on this site. I have run an Eaton Posi for the last 10 years but have had to rebuild it twice due to broken side gears. I just rebuilt it for the second time a few weeks ago and the original carbon clutches still look new.

 

With a 6 speed and 29"-30" tires I would go 4.88. I've had several 26" tire 6 speed F body and 4.11 wasn't enough. With the taller tires I bet the 4.88 would be perfect when you put it in 6th. With 4.11 at 80 mph in 6th I was around 1500 on a 26" tire if my memory is correct

 

If you have a T56 and launching at the track on slicks in a 4000+lb truck, I personally would not invest any money into the 10 bolt. Go straight to a 14 bolt. A manual trans is much much more abusive on the drivetrain than an automatic.

Mike