If your 05 GT is an auto transmission, it has 3.31 gears. If it is a manual, it has 3.55's.

 

Too bad you bought an '07 then Charlie.
In '08 they would have made the decision for you.
The Deluxe only comes with 3.31s for '08.

 

Not really any difference to worry about. My 07 GT Automatic came standard with 3.31's. Window sticker said 17 mpg city, 23 mpg highway. With the new-for-2008 testing methods, my 07 is now rated at 15 mpg city, 22 mpg highway.

I have 4.10 gears. I get about 15 mpg city (13 mpg if I flash the stall converter a lot, but 15 mpg before I replaced the factory torque converter, and 15 mpg now if I drive normal around town and leave my lead foot at home). My last highway trip netted me 22.5 mpg. So, even with steep 4.10 drag gears, I'm hitting the numbers the feds say a stock GT Automatic is good for.

In summary, DO NOT FEAR THE GEAR! You'll get much better acceleration in all gears. You'll be much happier driving every day. You'll be a lot quicker at the track. You can still get the same fuel economy as stock if you drive responsibly.

 

Sweet! Well then that's something I might think about in the future, as I know it's one of the cheapest for the most results performance-wise.

How much of a difference would I notice between 3.55 gears and 3.73? Would it make any sense for a DD to have 4.10?

Sorry, I just came over from the darkside (I had a 2001 Toyota Celica), and I'm new to cars with "power" (I didn't realize I didn't have that before until I stepped on the gas of a mustang in a test drive...the result? No Celica anymore...), and switching gears isn't something the import guys I was talking to were all about.

 

There's an Excel spreadsheet in this thread.

I updated it (red font columns) in the attached spreadsheet (below - rpms with differences.xls) to show differences between 4.10s and 3.73s, 4.10s and 3.55s, and 3.73s and 3.55s.

The differences become more pronounced at higher speeds in gears.
Keep in mind you're not going to hit a lot of those rpms during daily driving, I rarely go over 4k and usually keep between 1500 and 3k.

The part to keep watch for would be highway driving, and that's not bad at all - at 75mph in 5th, the 4.10s are only 350 rpms more than 3.55s.

Added:

I just traded my Scion tC in on an 08 Mustang GT manual with 3.55s, and everywhere I drove in the old tC, I am driving in the GT one gear lower. The Scion had obnoxious gearing and makes the numbers for 4.10s look like no problem for a DD. I'm looking forward to 4.10s.

 

yay! something else to spend money on!!! ha ha!

Seriously, that's so cool and thank you for outlining those gear sections for me! Some of my friends who persuaded me to look at Mustangs are big fans of gearing changes, and I'm beginning to see why.

I'm struggling between all the things I want to do to this car (like so many others here, at least I'm in the right place!) and being realistic about the fact that it is a DD, and that there are other financial constraints I have to pay attention to. also there's the fact that I live in an apartment for the time being, so there's only so much I'm comfortable doing in a parking lot!

But looking at all the mustangs out there, and all the aftermarket support, it's nice to know that I'll still be able to keep modding this one 30 years from now if I want!

Cheers, and thanks for the advice, you guys rock!

 

That's probably the # 1 reason I bought my Mustang. I know that if anything goes wrong I can get absolutely expert advice from the awesome gearheads on this and other enthusiast Mustang modding forums. A lot of people here are awesome with their knowledge - I know absolutely nothing about the mechanicals of cars, but really love learning.

Still... took me the past two months to wrap my mind around gears - glad I could help!

 

In M5's 4.10's are the conservative choice. 4.30's the aggressive,

In A5's they are kind of a strange beast. Several people have seen very little gain going from 3.31 to 3.73.
I believe this is because of the large gap between 3-4 (1.54 / 1.00).
This forces you into 4th earlier and yet the car doesnt really have enough gear to pull it effectively.
At the same time the lower overall gear ratio (0.71OD vs 0.675OD) results in the 4.30
which is what the A5 really needs having highway rpm like a M5 4.56 combo.
If I owned an Auto I would likely run a 4.30 with 28's on the street and 26.5 ET Street (26x11.50-17) on the track.
The other option would be to run 4.10's and one of the short 15" Slicks on custom 15" Wheels.
For that matter I really think the guys with an A5 3.73 combo should try a M&H 8.50/23.0 and custom 15's.

 

M5 as in the BMW M5?
A5 as in the Audi A5?

ETA: I see you're referring to the automatic and manual transmissions...

 

M5 as in Manual Five Speed
A5 as in Automatic Five Speed

 

I usually refer to them as TR-3650 (manual) and 5R55S (automatic).

 

That only makes sense if Ford made 3:55 a 'compulsory' option.

 

Double post...oops.

 

Yeah, I know what you mean Metroplex. I usually call the spark plugs the SP462's just so nobody gets confused.

 

Well it is when you buy a dealer specced car off the lot because you don't want to wait for a custom order and that one with the 3.55s was the only one of 5 manual GT 'stangs on the lot with the spoiler delete and no hood scoops. Just... er... speaking from experience.

 

yeah it still doesn't add credence to the conspiracy theory