Rear Axle Change for GT Automatic ??

You don't need the whole rear end just the new gears and the kit to install them. You can do them yourself, though it's not easy. Took me 3 days on my 05. Or you can avoid the headache and take it to a shop or to Ford. If Ford installs it they'll probably warranty it and could reprogram the ecu for you. If you take it elsewhere buy a tuner to reprogram the ratio yourself, it just takes a few minutes.

Just to let you know 3.55s will be a waste of money. Trust me go with 3.73s or 4.10s. You will actually gain some gas mileage around town but might drop 1-2 on the highway. It will equal out and you shouldn't notice a change in gas. I have 3.73s and my 01GT is only a 4 speed automatic yours is a five speed auto. I still get 18-20 mpg mix highway/around town, with stage II cams and I don't drive like a grandma. I actually have 4.10s that are going in this month. Once again whatever you do go with 3.73s or 4.10s.

The S197 GT's (2005-2010) are heavy enough cars that 3.55's aren't really enough gear, especially for a lazy automatic transmission. 4.10's are really the best all around gear for a GT Automatic. The dealers can't adjust for that much gear change in the car's computer, however, so you'll need a handheld tuner (i.e. Diablosport, SCT, etc.). As already mentioned, your in town fuel economy will improve while your highway economy will drop a tad bit. But, your fun factor will go through the roof as 4.10's really wake up your car's performance and drivability.

Is there a tuner out yet that is compatable with the 10s ECU?

I ordered an SCT X3 (w/torque,performance and race tunes) from Bamachips and it supposedly shipped yesterday.

Last time I talked to a dealer about gear install, they wanted $900 to do it. If you can pick up a whole rearend off ebay cheap enough, it's a breaze to install if you've got tools and a place. I've done it on old Broncos and Ford trucks many times. I've also installed gears a couple of times, and it's pretty tough to get right. You could always install a rear end, then buy a tuner to change your speedo.

Teleplayer said:

Thanks for the input. I'm still partial to getting a complete rearend with the 3.73's already in place. That way I don't have to play with the gears and preload and all that stuff. Plus I'd like to be able to setup the ECM without having to go to an aftermarket setup. It might be possible to fake out the ECM if you can compensate with a smaller tire circumference.

Five-Oh Brian: Have you driven the 3.73 in the auto or are you just basing it on the fact that you have the 4.10's on yours? Did you go from 3.33 to 4.10? Seems like a huge change.

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The factory computer can not be adjusted enough to fix the speedometer after a gear change. Get an aftermarket handheld tuner.

I've had cars with 3.73's, I've driven S197's with 3.73's, etc. It's not enough gear, IMO. My car came from the factory with 3.31's. I had the 4.10's installed when the car was 2 weeks old. I had 4.30's in my '03 Mach 1, 4.10's in my '93 T-Bird 5.0L LX, 3.73's in my '89 Mustang 5.0L LX, 3.90's in my '68 Pontiac GTO, etc. All of these cars had much lower-numbered gears initially, then I chose steeper gears.

Search the forums for a consensus and you'll find some people who installed 3.73's in their 2005+ Mustangs who are now wondering if they should have done 4.10's or 4.30's. You will likely never (or very rarely) hear anyone with 4.10's or 4.30's wonder if 3.73's would have been better.

3.31's/3.55's aren't enough gear, 3.73's are OK, 4.10's rock, and 4.30's are a bit too much for some, and 4.56's are overkill for 95% of the Mustang owners out there.

Fuel economy doesn't really drop too much on the highway due to the wonderful overdrive transmissions we have. Fuel economy in town is basically unchanged with steeper gears, though, as the car accelerates easier with less throttle position than with the lame 3.31 or 3.55's which lug the engine from a stop (or low speed). On the flip side, drivability and pure enjoyment is increased ten-fold with steeper gears, and that's why so many people have switched to 4.10's.