Trek724

I have a 12 GT auto with the 3.15 rear gear. If I go to a 3.55 will I see a noticeable difference in acceleration. Also I will not be tuning the car at this time.
Thanks for any input.

MrSandman

If you want a noticeable difference.. id think 3.73s would be ideal. I cant speak for a 12 auto, but with my 07 auto (totally different), the 3.73s made the car fun again. With my 11 manual, the 3.73s make first a little brief.. but second is a monster.. and fifth.. 60-100 in a heartbeat.

Mudflap

Gear swaps aren't something you want to do more than necessary IMO. Getting a good install can be a challenge. Agree with the above, if you're going to do it, make it count and go 3.73s.

terrellble

Don't you have to get a tune to put in new gears on an auto?

scottmach

Yes because 3.55's are not an option with the autos.

scottmach

Not much of a noticeable difference. Ford put 3.15's in the autos because they have 4.17's as a first gear in the tranny. A tune would make a night and day difference but you stated you won't be doing that.

Mishri

you'll have to send your PCM to ford if you don't tune it, the charge a butt load for it, i got a quote for $800 on my '07, unless something changed since then.

noticable? maybe a bit... for optimal 1/4 mile times you want either 3.31s or something beyond 4.10s.

I like my gears, my gas mileage went down the crapper but they are fun

I had a guy do mine that had done other people's and did a good job. Ask around at local places who has done gear swaps on 05+ mustangs and is satisfied there is no whine.

Trek724

I did not know I needed a tune for a gear swap. In that case I may consider a tune, I'm just a little hesitant because of some of the issues I've read about with aftermarket tunes.
But the increased performance I would definitely welcome.
Mishri what's your combined gas mileage with 4.10s?

SilverHoss

IMO Ford put 3.15 in these cars to try and get a better MPG out of these cars, not because of the tranny gearing. Reason I say this, my wife's KIA Sorento seems to accelerate easier at light throttle, especially off a dead start than my GT Mustang ! I wondered why, so I looked up the specs, and the KIA uses higher ratios in the auto tranny than the Mustang. Check this out:

KIA Sorento 3.5L V6:
1st 4.651
2nd 2.831
3rd 1.842
4th 1.386
5th 1.000
6th 0.772
Final drive 3.195:1

Mustang:
1st 4.17
2nd 2.34
3rd 1.52
4th 1.14
5th 0.87
6th 0.69
Final drive 3.15:1

As you can see, the gear ratios for my wife's KIA are much more favorable for acceleration than my Mustang GT 5.0. But KIA can run those because they Sorento uses a smaller 3.5L v6 to our 5.0L v8 and so they can get good acceleration out of a heavy vehicle and still have good MPG.
Sorry for the long post, but it goes back to my opinion that Ford used a 3.15:1 rear end ratio for better MPG. And yes, hell yes I think 3.55 gear would make a nice difference in acceleration and make the Mustang feel lighter and take off faster off the line IF you can get it to hook up!
3.73 might be a bit much. Would not go 4.10 with the auto for sure.

scottmach

So now we're comparing KIA's to Mustangs? FYI My auto with 3.15's and some suspension work will absolutely roast my tires if I just nail it. They're plenty for me.