Be ready to have more differential noise. Lots of complaints with the 373's. With a stock 5.0 the 355's and MT82 seem to work nicely for street driving.

 

 

What is your present gear ratio and what is your objective in changing to 3.73's? There isn't any perfect ratio for all conditions so it would be helpful to know what you wish to achieve. That said 3.55 is probably the closest thing to the "Goldilocks" choice.

 

 

There are no stock 4.11 gears in your car. 3.73 is the most aggressive and 3.31 is the most relaxed, with 3.55 in the middle.

 

 

The 2012 GT came with 3.31 on the manual and 3.15 on the automatic. The lower the number, the better the gas mileage because the engine will be running at lower RPMs relative to the road speed. But the tradeoff is reduced acceleration and torque. The reverse is also true... going to a higher ratio will reduce gas mileage but increase acceleration.

If mileage is your primary desire then none of the higher ratios (3.55, 3.73, and 4.11) will do what you want. If you have an automatic, there is no lower ratio except for the 2.73 that was used in the V6 models. If you have a stick, you could go down to the 3.15 or the 2.73 for a slight increase in mileage (about 1 mpg) but the difference probably isn't worth the loss of torque. In fact, you might actually lose mileage overall when you start pushing the accelerator more to make up for the lost acceleration. Besides, if mileage was the primary consideration, why buy a V8 in the first place?

 

There are s-loads of threads on this, try the search and you should have plenty of interesting reading with dozens of different opinions and all kinds of lengthy discussions.

My opinion, based on reading a lot of those other threads and my experience with 3.73's in my 2010 (it's a 5-speed and yes it's a little different): keep your 3.31's

 

He hasn't said why he wants to change but unless it's for drag racing, I agree. 3.31 gears are a big reason I have averaged 23 mpg over the 130k miles I've owned my Mustang.

 

The OP is looking for better gas mileage and appears to be confused about gear ratios.