3.55 or 3.73 gears?

You, in theory, might get slightly better mileage in town with the 3.73's as it would require less "effort" to get the car moving.

Between the two, the difference, at freeway speed, is only a few hundred RPM. Which the six-speed will help with.

If you were doing a lot of highway driving, the gears would affect fuel economy more. If you are just going to be driving slow in stop and go city traffic, changing the rear-end gear probably won't have much of any affect, since you're not in high gear in the city, so you'll probably just end up shifting the transmission into a higher gear at a lower speed. If your driving conditions and speeds are just right, being able to upshift a little earlier could slightly improve fuel economy.

3.73! the previous posts are right. Since you are in stop and go city traffic, you probably will not be running in 6th gear much, and that is where, as far as mpg difference is. 3.55 will be able to run lower rpms in 6th. You will appreciate the power difference from the 3.73. But only go with 3.73 not 4.10s, I personally think 4.10s are on the torquey side for street cars, but every driver is different

I love the 3.73 in my car. I was fortunate enough to drive a Mustang with 3.31, 3.55, and 3.73. For performance, the 3.73 give much better response.

I had 3.55s in my '07, and now have 3.73s in my '11. For city driving, the 3.73s are way more fun! If you don't have a good tire in the back, you will have traction issues if your right foot is a bit heavy. I can spin the tires in 1st with just hard, roll-on acceleration.

Regarding highway mileage, I have found this to be true for me:

'07 GT, 3.55, 5-speed stick, 70mph, 2,250 rpm = close to, if not...30 mpg. Strange, I know.
'11 GT, 3.75, 6-speed stick, 70mph, 2,250 rpm = at best, 22 mpg, MAYBE 24...but it took forever to register.

I have no idea why the huge difference in MPG if both cars turned the same rpm on the highway. Both had K&N drop-in air filters. Stock tires. Windows up, AC on or off.

If you like to DRIVE, you'll have much more fun with 3.73s. The rpms are not crazy at all on the highway (2,250 at 70 in 6th). Around town I drive my car like a 5-speed and with my VERY short commute of under 3 miles one way, I get between 11.5-12.5 mpg. That's not driving like a crazy person either. Half the commute is 35mph or less. If I use 6th gear around town (pfthhh), I loaf along around 1,200-1,500 rpm and can see 15mpg or so.

I just read that you have a V6, so ignore my MPGs totally.

Hope that helps.

3.73s. You won't see much difference in mileage, but will be happier with performance.