I went into the site mention above, this is what I got with my tire size set up and the 3.69 FD


am i suppose to feel my car pull faster from going 3.5 to 3.69 FD or what should I be expecting from the FD swap?

 

The point of going to a lower gearing on the rear end (higher number in terms of FD ratio) is that you would have more pull in each gear, thus making your car accelerate quicker without actually having more horsepower. The problem is that if you went from 3.5 to 3.69, that's an almost insignificant difference. You won't likely notice too much of a difference unless you go to something like 3.9 ratio rear end. On the flip side of things, when you go to lower gear ratios, your fuel economy on the highway will go down a bit and your engine and transmission will take slightly more abuse due to them spinning about 10% more per mile traveled in comparison to a stock-geared car (nothing you'll really notice though, assuming you keep up with your car's maintenance). Your speedometer will also be off by that same % and your odometer will be off by that % for each mile that you travel after doing the swap unless it turns out that our cars actually do pickup the speed reading from the rear wheels themselves rather than from the transmission output shaft.

Most cars use the transmission output shaft though because your rear wheels spin at different speeds when you are cornering, so it's harder to tell the actual speed when you get conflicting numbers from each wheel.

 

I should of have gone 3.9 then, do you know which year and model carry 3.9 stock? is it a 370z?

 

Just get one of these and be done with it:

http://www.nissanraceshop.com/produc...ion-381000c760

http://www.z1auto.com/prodmore.asp?m...ne&prodid=2833

From all accounts I've read, those 4.083 gears make a HUUUGE difference.

Here's one guy's review of them:

https://g35driver.com/forums/drivetr...ive-gears.html

You're looking at $500-600 in parts/shipping and a few hundred bucks for installation, but you will probably be floored by the difference in acceleration your car will have with the 4.083 gearing.

I'm not an expert on all of the vendors who might make different gear sets specifically for the G35, so be sure to do some comparative research of other aftermarket FD gear manufacturers an decide which you think is best. My links above are just to point you in the right direction and get some ideas of your options.

Here is a link to an aftermarket 3.9 Gear set, but it costs more than the 4.083 ratio set:
http://www.g35parts.com/Central-20-3.../cen20z33g.htm

 

Came across this thread b/c I was searching about swapping in the G37 rear diff. Figure I'd try to clear up some of the questions I see in this thread.

Speedo: Vehicle speed is determined by the ABS sensors in the wheels, there is no transmission based speed sensor (like on the old 240SX's). The speedometer actually gets the vehicle speed from the VDC/TCS/ABS control unit via CAN communication. That said, changing out the FD should have no affect on your Speedo accuracy.

G37 Diff: It would be pretty obvious if you got an Auto diff because the input flange is completely different (3 bolt) from the MT and all 1st gen diffs (which are 4 bolts).

 

How exactly did you test that theory? Because I know for a fact that my speedometer was off, and a local member who changed out the FD on an 06 noticed the change in his as well.

 

Going by the FSM. For reference, check the relevant sections of DI, BRC, and EC.

Mainly pointing out that the wheel sensors are used for the speedo, not a sensor in the trans as someone was suggesting. I don't know why your speedo was off, in theory it shouldn't be. Unless the ABS control unit is doing some odd-ball calculations.

 

I'm not a mechanic so I'm not going to try and decipher the FSM's or schematics, I'm going off of what I've experienced. When I changed out the stock differential (3.5 VLSD) with that of a G37's (3.7 VLSD), the speedometer became progressively more inaccurate with an increase in speed. This inaccuracy was confirmed on two separate dynos (since the roller speed didn't match the indicated speed), GPS, and comparing travel speeds with others in a group.

The only variable that changed in this situation was the FD gearing. Since returning to a 3.5 gearing, my speedometer is once again accurate. Like I said previously I'm not the only one that this has happened to. Another member with the same year Coupe, switched to a 3.7 as well and has had the same results. And as far as I could tell, all other systems on my car have worked properly.

Did you change your FD ratio?

 

Haven't changed mine yet. Came across one and started searching around here to see what the pros & cons were. Mainly wondering if it would make that much of a difference.

For your speedo, with the 3.7 gears, was it reading high or low?

 

It was reading high. At roughly 80mph, the car indicated it was doing 90. Going to a 3.7 isn't much of a difference but still noticeable.

 

Do these fit AT? On the second link it says for manual transmission

 

I was doing some research in regards to upgrading my '06 open differential and came across a couple of your threads. From your experience what would you recommend for an OEM LSD swap and what modifications would be involved?

 

^PM replied.

 

hey i have a automatic 2008 g35 that the differential is making a humming sound. the mechanic told me that it is better to replace it. which year i have compatibility without messing anything? can i use one from a G37?

 

Wow.... this old thread is back from the dead.

Wait and see if the differential noise gets worse or if you start noticing any loss in power or fuel economy. I would not recommend replacing the diff just because some mechanic told you to unless he has hard proof that the current one has failed. Is the fluid in the diff filled to spec? It could be something like worn bearings in the U-Joints (ours are no-maintenance sealed units).

How do you know for sure that it's the differential that the noise is coming from? Did your mechanic lift the rear end and run it through the gears and clearly hear it coming from the diff? I have a tool called a "Steelman Chassis Ear" that I use for isolating the source of noises and vibrations such as these that can only be replicated while the vehicle is actually moving. It works really well and you can do it all from safely inside the cabin while driving.