I am trying to do a 6l90 w/3.90 gear. I learned in a different forum that this might not ba a good combo because of the 1st gear in the tranny. Can anyone elaborate? And how do I find the most performance oriented gear to that would work with this tranny. Looking at at possible 2500 stall

 

In what? what weight and gear?
It is a different ballgame than the 4L60 ratios for sure.
If you're building a third gen, you may want to look at what the 5th gen got with the 6L80. They were heavier with slightly taller tires, so a stock Gen IV LS V8 / 6L80 combo into a third gen with similar rear gear would drive out nicely. Drive one.
I have a 5.3 truck w/ 6L80, 3.08 gear and 31.5" tires. I skipped over the 4L60 trucks on purpose and have no regrets. I could run this combo in a third gen w shorter tires and be happy for drivability. The 1st gear is gling to make the ideal stall lower than what works with a 4L60.

 

Sorry about that it's a 91 Camaro RS stock form right now it has a small block 350 with a 700R4 342 posi 4th gen rear disc brakes proportionate valve upgrade and it's going to have C6 18 inch 10 1/2 wide in the back 9 in the front chrome five spoke wheels absolutely no racing no AC. Soon to have an all aluminum LS3 the 480 horsepower version and like I said I planned on using aa 3.90 gear

 

 

You'll be going from a 3.06 first gear to 4.0x.
That's similar to going from 3.42 to 4.50 rear gear.
Definitely skip the change from 3.42 to 3.90.
The LS3 will pull 27" tire, 3.42, 0.67 sixth no problem.
Should be a very enjoyable upgrade build.

 

Thank you my man I appreciate the information