Stock for the 2003-2006 (sedan and coupe) and 2007 (coupe only) automatic transmissions (5AT) is a 3.35 final drive ratio. This is designed to keep cruising rpm speeds down inside the optimal MPG.

I have been getting 25-29 MPG HWY after doing several mods. Before the mods i was around 23-24 MPG.

I wanted VLSD (viscous limited slip) but mostly I wanted better acceleration. All 6MT transmission from 2003-2007 have the 3.53 FD. VLSD is optional for some. 350Z share the same differential setups.

I found a differential from a 2006 6MT so that I would have the 3.5 final drive and VLSD. I did the swap and here are my findings:

Install difficulty was 2 of 4. Not too difficult, just big and heavy.

Advantages: Noticeably quicker acceleration. Freeway on-ramps are a lot of fun now. Handling in turns is much better and with VDC off the electronic brake slip limiter does not kick in like it used to. Torque output is a much better range for climbing up hills as my speed was constant and I never had to kick down to a different gear as I previously would in the same spots.

Disadvantages: Rpms at 80 MPH increased from 2875 to about 3075, an increase of roughly 200 rpms. With the increase in rpms cruising at the same speed will lower your gas mileage by 2-3 mpg. Before, the best cruising speed was 78-82 MPH, now I have found that I can keep the same gas mileage but need to reduce my speed and the sweet spot is now 73-78. This is roughly a 5 mph difference.

Results would be much more dramatic for all things listed above by going to a more aggressive ring/pinion. 3.69 (3.7) / 3.9 /4.08 (4.1).

Many people who stay NA and are looking for more fun out of there 6MT swap to the 3.9 FD or even the 4.08. Those are generally accepted as too aggresive for 5AT and most 5AT guys go with the 3.5 or 3.7. A few have bumped up to the 3.9.

Each step to shorter gearing (taller numbers) is an increase in about 5% of gear ratio. Accordingly, 5% change in gas mileage is reported + a lot more heavy driving due to the fun factor.

I am going to be doing the swap from a 3.3 open diff to a 3.7 vlsd here is a good site to find the "carrier assembly"!!
http://www.partshotlines.com/
I got my 2008 3.7 FD vlsd off a S model for 450 shipped!