Is there a huge difference between them? I'm getting a GT with manual transmission and can't find a 3.31 at the dealer to test drive.

 

There are three big differences in the two. I think the 3.31 housing isn't the same as the 3.55 housing. So if you decide to go to 3.55 or 3.73 later you'll need to replace the whole housing. Not a big deal as they aren't really expensive. Aside from that the 3.31 will give slightly better mileage and lets you use 1st gear longer. But the 3.55 lets you come out a slight bit stronger but with a shorter 1st, making it necessary to go to 4th sooner on a drag strip. You probably won't notice all that on a test drive. You have to live with one to see and feel the differences. Coming from the V6 to the GT the extra power will make all this moot until you live with one.

 

3.55 should accelerate faster, 3.31 will give a little better economy on the interstate.

 

I test drove both and honestly couldn't tell the difference. I ended up buying one with a 3.31 and haven't thought about it since.

 

I test drove a 14 GT with 3.31s and purchased a TP car with 3.73's. Huge difference, the 3.31 cars feel sluggish by comparison. You just rip though the gears with 3.73s.

I think 3.55 is a "best of both worlds" option. I track my car and at some tracks I run out of 3rd gear with just about 100 years left to the brake zone. 3.55's are better suited for road courses due to slightly taller gearing.

 

I have no plans on driving my car at a track or any kind of racing. Sure,I'll rip through the gears every once in a while() but I'm thinking more in the mode of a classic Gran Touring car.
I think I'll keep looking for a 3.31 in addition to the 3.55.
Thanks to all for the info!

 

If this is your first modern 5.0, I think you'll be very happy with the 3.31. I have it on my '12 vert since it is the only diff available on the verts. 1st is still too short on this car, I can't imagine it with a higher ratio axle. (I'd be bouncing the rev limiter even more often!). I get 23 MPG in combined highway/city with more of my driving being highway. I always run premium. Ran a couple of talks of regular but there is a very noticeable difference.