Rear gear selection for LS engine, opinions please.

[Tim john---]

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---

[OBeer-WAN-Kenobi]

Originally Posted by Tim john---

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.

[Evan Iroc-Z]

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.

[LS1-IROC]

A stock LS1 with a 6 speed with 4:10's is a match made in heaven. I ran that set-up for years in my IROC. Could pull down 29mpg on the highway on long trips. Once I put the blower on I had to knock it down to a 3:50 so I could still use 1st gear.