Rear gear selection for LS engine, opinions please.

[Tim john---]

Good afternooon all, I would like some honest opinions on rear gear selections. Car in question is a 69 Firebird convertible with (will get) a stock 2004 GTO LS-1 engine and most likely a Tremec T-56 magnum six speed. Car will be a very nice stock appearing vehicle with no track duty in mind (for now). Project is just starting to get off the ground, everything is disassembled. Car will be DSE mini tubbed (tubs currently installed). I have two rear ends in stock, a grey iron open (non-posi) 2:56 and a Nodular posi w/ 3:36. I have to alter one of the rears for the mini tubbs (re-locate the spring perches). The 2004 GTO's all came w/ a 3:42 with a six speed I belive so would I be best off with the 3:36 or step up to a 3:55 or a 3:90. Keep in mind this car is not being built for track duty, just a really fun street car with stock 2004 GTO performance with reasonable fuel mileage. Thank You.

Tim john---

[OBeer-WAN-Kenobi]

Originally Posted by Tim john---

Good afternooon all, I would like some honest opinions on rear gear selections. Car in question is a 69 Firebird convertible with (will get) a stock 2004 GTO LS-1 engine and most likely a Tremec T-56 magnum six speed. Car will be a very nice stock appearing vehicle with no track duty in mind (for now). Project is just starting to get off the ground, everything is disassembled. Car will be DSE mini tubbed (tubs currently installed). I have two rear ends in stock, a grey iron open (non-posi) 2:56 and a Nodular posi w/ 3:36. I have to alter one of the rears for the mini tubbs (re-locate the spring perches). The 2004 GTO's all came w/ a 3:42 with a six speed I belive so would I be best off with the 3:36 or step up to a 3:55 or a 3:90. Keep in mind this car is not being built for track duty, just a really fun street car with stock 2004 GTO performance with reasonable fuel mileage. Thank You.

Tim john---

I have a complete pull-out from a 2004 GTO, including the GTO T56 in my 69 Camaro. The engine is basically stock except for CAI, LT's and a tune.

I'm running a 4.10 Posi and it's a hoot! I think I'm running about 1800-1900 RPM in 6th gear at 60mph. I'm averaging 20 mpg. My best tank has been 21 with not a lot of difference if I drive all highway miles. That points to my gearing being a little bit short I believe. I can easily start out in second gear. First gear isn't worthless by any means but it's not as useful as it could be.


That being said, I'd like to try just a wee bit taller gears. Maybe a 3.90. However; I"m waiting until I get new tires that are to be about 2" bigger in diameter to see how it feels then. I may just stick with the 4.10's. The torque on the gear reduction is worlds of fun and with a double overdrive transmission like the T56 you can pretty much have your cake and eat it too. Therefore, I think that the absolute tallest gear I'd ever go with is a 3.73. If I were you, I think I'd look at a 3.90 though. I wouldn't talk you out of a 4.10 either; depending on your planned rear tire diameter and see below....

The only other consideration is whether or not you decide to go with the close ratio Magnum (if you're not sticking with the GTO transmission) I think it has a shorter overdrive.

[Z06vette]

I have run 3.73 & 4.10. Prefer the 4.10 It really keeps the motor in the powerband, better than the 3.73s. And if you ever getting into autocross, you can run most tracks in 2nd gear w/4.10. With a 335/30/18 I'm turning roughly 2100 @ 80mph. 22-24 mpg highway.
Scott

[Vince@Meanstreets]

on a stock gto ls your gonna like the 3:55's. we have run 3:42's and it wants to be lower gearing. try the 3:36's since you have them then change them out if you don't.

[OBeer-WAN-Kenobi]

Originally Posted by Vince@MSperfab

on a stock gto ls your gonna like the 3:55's. we have run 3:42's and it wants to be lower gearing. try the 3:36's since you have them then change them out if you don't.

Really? I think 6th is going to be pretty hard to use with that tall of a gear. Mine doesn't like anything much below 1600 RPM (in 6th) one bit. Could be the tune though I suppose.

What kind of MPG are you getting with 3.55's Vince and what tire diameter?

[57hemicuda]

You'll have double overdrive, go for the gusto. I'd run 3:89 or 4:10's with that trans. I drove from hear in Maryland to Tenn. in my buddies 66 Chevelle, LS1, T56 with 4:10's, the car never went into 6th gear unless we were doing 90mph, always lugging when under 90mph.