I'm going to purchase a 2015 Mustang GT convertible. When looking at dealer inventory I noticed some have a 3.55 Limited Slip Rear Axle and some have a 3.31. What is the difference? I like the power and sound of the GT but would probably never race it.

 

It changes the feel and drivability of the car. A 3.31 is a good daily driver set as you will get better mileage but at the loss of acceleration. My 09 has the upgraded 3.55 ratio from the factory and it is noticeably quicker than a 3.31 car. The gas mileage difference really isn't that big plus with the newer six speeds double overdrive i doubt you'd even notice unless you do mostly stop and go. There are plenty of calculators out there that will tell you the differences in engine RPM at various speeds and some even calculate gas mileage differences to help you decide.

Also if the 3.31 is Non Limited slip its worth it just to add that alone aftermarket LS is expensive and it makes the car much more tame on wet/ inclement road conditions.

 

A 3.55 rear axle ratio will give you slightly better acceleration at the expense of slightly lower MPG.

 

All GT's have had a Ford Traction-Lok Clutch type LSD as STANDARD since the 80's and V6 has had it STANDARD since 2011 as does the new Ecoboost.

Also another interesting tidbit is all Mustangs have had 31 Spline Axles since 2011 as well (may have actually become std. on GT sometime between 2005-2010). In the 60-70's 31 Splines axles only came on Big Block & Boss cars ... all others were 28 spline.

Get the Performance Package and you get a Torsen LSD in place of the Ford LSD.

Doug

 

This. Numerically higher axle ratios will get you off the line quicker at the cost of some fuel mileage and top speed. Numerically lower axle ratios will sacrifice off the line performance for fuel mileage and top speed. Since you aren't planning on doing any sort of competitive driving, the 3.31 gears should be fine.

 

Sidenote, the Getrag is a single overdrive.
5th gear is 1:1