Rear gear selection for LS engine, opinions please.
 

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.

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

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?

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.

You need to run around a 4 10 to be able to use 6th gear at like these guys are saying,they are right on with the T56.

Thanks guys, very much appriciate the opinions. I have a question then, why would the factory only offer the 3:42 for the 2004 LS GTO. Engineers do things for a reason and I would think their gear choice would have been optimized for that engine (LS). I also would not be going for the close ratio Tremec, most likely the .063 6th. gear version. The transmission would be the aftermarket Tremec T-56 not the factory 2004 GTO issue six speed. I want to have the floor shifter come through the floor console to look factory correct and that is not possible with the 2004 factory six speed plus I do not have either, need to purchase. I do have the factory automatic that came with the engine but will not be using it, hopefully sell it to help offset the cost of the manual unit. For now I would be running my original HURST wheels in 14" X 6" width with a 26" tall tire, also have some Rally II rims in 15" X 7" as an option and when the time comes I may have to get all new wheels and tires but uncertain on sizing, one thing I do not want is a thin side wall or a deep dish from the front face. I would rather have the back spacing than the deep dish look. I would like the car to look realitively stock from a side view and from the rear a nice thick tire foot print would be desirable.

Tim john---

The engineers probably geared it for mileage only. There is probably some sort of "gas guzzler" tax that they could avoid with 1/2 more mpg or something. Who knows in Australia. If you check the GTO forums you'll find that a lot of people change their gears.

I believe the .63 6th gear is the close ratio. Stock GTO is .56. This would tend to make me lean toward the 3.9 or possibly a 3.73 (max) as opposed to the 4.10. There is a calculator online where you can put in all six ratios from your t56, rear ratio and tire diameter. You can play with that. It will show you the speed in each gear for a given RPM or vice versa. In general, pick your desired cruising speed (for a driver car I use 60mph) and your best mileage; from what I've read, is a couple hundred RPM before your cam wakes up. That's not necessarily your best performance though, in fact, probably isn't.

Don't pass up a deal on a GTO t56 if you find one. It's as simple as buying an easily installed offset cup and an F body shifter to convert it to the "stock" location. The GTO t56 isn't as good as a magnum but has better synchros than an F body unit and is absolutely fine for stock or near stock power levels.

The car that you're putting this in its about 500 lbs less than the GTO it came out of and shorter gears are a lot more fun so if mileage of 20+ is good enough for you stick with the fun gears, you won't be sorry.

On a side note, in case you didn't know, you'll need a manual engine harness our you'll need to modify the automatic one and you'll need your computer changed to the manual transmission programming.

I agree with Vince, but want to say it is all in what you want out of the car. If you are looking for a cruiser and mileage, then go with the 3:36 and see what happens and if they don't work then go to the 3:55.

If you are looking for acceleration and are not concerned with mileage then go with the 3:90 or 4:10. As stated it will keep the engine happy and give you some good get up and go.

I have 3:42 with a 5sp, not on an LS, and it was a good balance for me. Gave me a little pep and also gave me good mileage.