Best route for gear change.

After posting on here before about changing my stock 3:15 rear, I decided that should be my next mod. I notice you can buy the 3:55 complete rear for $899, they also sell the complete PP rear in 3:73 Torsen for $1899 (not sure if this would bolt directly onto an Auto car). I'd like 3:73 but not for $1899. My question is, would it be better to just get the 3:73 ring and pinion and change them out in my stock housing at a dealership? Any idea what a Ford dealer would charge for this? Or should I just go with the complete 3:55 housing for $899 which my friend can help me install. As much as I'd like the 3:73 rear, I just can't see spending an extra $1000 for the complete rear.

If you have not driven a 6A / 3.55 car, go to a dealer, drive one, and then decide if the difference is worth the expense of the swap.

If you do decide to move forward with the swap, just do the 3.55 ring and pinion.
From a drivability standpoint, I do not think you would that happy with the 6A and 3.73 rear gear, the 3.55 is plenty...IMHO.
See if you can find someone that owns an 6A S550 with the 3.73 and go for a ride.

Do not take it to the dealer, they will rape you.
Find a well known performance, driveline or transmission shop that routinely does gears and knows what they are doing.
The entire install and setup..backlash and preload etc. needs to be spot on.

Reach out to our forum members and talk to Mustang club members in your area for some shop suggestions and personal experiences with gear swaps.

A proper gear job should be free of vibration, whining or whirring sounds, and any other unusual metal to metal rubbing/contact sounds.
Sometimes the gear sets themselves will have an inherent manufacturing defect that will not reveal itself until the the first drive.

For best results, use a Ford Racing ring and pinion and install kit.
Now, you will need a tune.
Lund seems to be one of the best in the business, especially for the automatics.

You cannot change out the gears in the 3.15 housing since it is to small. You would have to buy a whole new rear. Changes are made through the BCM, not the PCM. This requires the Whipple flare tool. It will be a lot of money spent for zero performance gain. I strongly recommend a converter change.

I don't believe the 3:55 ring and pinion will fit on the 3:15 set up.
http://www.levittownfordparts.com/ford-racing-mustang-inch-ring-gear-pinion-p-3025.html

Knowledge deficit on my part...thanks guys!
Definitely not worth the cost.

2015 Silver GT said:

You cannot change out the gears in the 3.15 housing since it is to small. You would have to buy a whole new rear. Changes are made through the BCM, not the PCM. This requires the Whipple flare tool. It will be a lot of money spent for zero performance gain. I strongly recommend a converter change.

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That's what I heard, at least buying the whole housing guarantee's me everything is lined up inside correctly, since as EJS2016 said, it's a job best done by someone who's done it many times before. So is the general consensus that I won't see much difference going from 3:15 to 3:55 on an auto? Money better spent on a Tune and PMAS CAI?

Try the largest Wrecking yard

in your area. They can look on the national net and get you a Auto Diff unit with the 3:55 gears. There are a few of those already in the system from parted out low mileage cars. That will save you the hassle of disassembly and adjustments of changing the gears. Getting the Auto unit will also let you do a one for one swap and they may take yours to keep the price down. I'm just sayin....

2015Etrac said:

That's what I heard, at least buying the whole housing guarantee's me everything is lined up inside correctly, since as EJS2016 said, it's a job best done by someone who's done it many times before. So is the general consensus that I won't see much difference going from 3:15 to 3:55 on an auto? Money better spent on a Tune and PMAS CAI?

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You won't see much of a difference at all going from 3.15's to 3.55's. The car will "feel" a little faster but in essence won't be faster. I went 11.2 N/A with my 3.15's so they're plenty of gear. Start with a good tune. That will make a night and day difference.

2015Etrac said:

So is the general consensus that I won't see much difference going from 3:15 to 3:55 on an auto? Money better spent on a Tune and PMAS CAI?

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I'm considering this as well. That is the consensus but I don't understand how you can go from 3.15 to 3.55 and not pick up a few tenths in the quarter mile. It seems like the heavier S550 would benefit from this or Ford wouldn't offer it as an option.

SVT Wylde said:

I'm considering this as well. That is the consensus but I don't understand how you can go from 3.15 to 3.55 and not pick up a few tenths in the quarter mile. It seems like the heavier S550 would benefit from this or Ford wouldn't offer it as an option.

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I've read that it's because the 3:15 will keep you at high RPM's in 3rd on the 1/4 mile, but the 3:55 will go into 4th? I could be wrong, and I'm far from an expert, that's just what I've heard.

2015 Silver GT said:

You won't see much of a difference at all going from 3.15's to 3.55's. The car will "feel" a little faster but in essence won't be faster. I went 11.2 N/A with my 3.15's so they're plenty of gear. Start with a good tune. That will make a night and day difference.

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I wouldn't mind hitting the low 11's. What does your car have done to it?

SVT Wylde said:

I'm considering this as well. That is the consensus but I don't understand how you can go from 3.15 to 3.55 and not pick up a few tenths in the quarter mile. It seems like the heavier S550 would benefit from this or Ford wouldn't offer it as an option.

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It makes perfect sense. More gear doesn't always equate to a faster car. You have to factor in the gearing in the transmission as well.

2015Etrac said:

I wouldn't mind hitting the low 11's. What does your car have done to it?

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I ran the Boss Intake, 4C converter, full exhaust, all on the stock 3.15's.

If you switch the whole unit I would try to go with the iron housing. You need to be careful though because the manual and auto have a different flange on the diff.

I have a low mile Torsen diff with 3.31 gears installed, for sale if your interested.

I swapped 3.15 to 3.55, the whole housing. Love the daily drive and dropped .2 in the 1/4 mile. I was already shifting into 4th because 26" tires which were faster then 28" to begin with. Ran 11.4 first night with new gears. Found another guy to do an even swap so just paid for install. My speedo was also spot on so I didn't need any other adjustments.