[Aussie Mike]

I'm running 3.91 in my CR. I reckon they work great with the 6 speed box. 3.7 might be better for a torque monster LS2 or LS3 motor but my little 5.7 likes the 3.9 gears.

The 6 speed has a tall first and second gear so they are both very usable with the short diff gears.

The 3.9 gears with the T56 2.66 first gear has you shifting 1st to 2nd at 70KMH @ 6000RPM. That's with 315/35/17 rear tyres which are a bit under 26" tall

Cruising @110 in 6th has you ticking over at a nice economical 1700RPM or 2500RPM in 5th ready for a quick getaway.

The same setup with a T5 sees you shifting to 2nd at about 55KMH @ 6000 RPM

Cruising @110 in 5th has you sitting on 2200RPM with the T5.

The 3.692 or 3.583 might be a better choice with the T5.

Cheers