Ahhh...forgot you did not have a handheld SCT 2/3 since you did not install anything!
What gears are in the car you are looking at?
Understand your frustration with Carmax but there is no way they could make you an offer without it being there in front of them...just imagine how many calls they take a day with "hey man, how much is my car worth"!
I think the 12 has stock MT ratio. 3.31?? Since anything else would be listed on the window sticker under "Optional Equipment". I.E. 3.55, 3.73....
I wasn't just calling up Carmax saying "hey man, how much is my car worth?" I was there, in front of them, well prepared, with my laptop full of hi-res photos, and a folder full of all the paperwork ready to give detailed information. It wasn't like the guy was busy, there was 2 customers in the whole place.
If it was me, and someone walked in and said they have a Mustang GT with only 1300mi, and they had a laptop and paperwork in hand, I'd take 2 minutes to see if they got something I could make money on! If yes, after the 2 minutes, then I'd say ok, now go and bring the car in so i can take a closer look. I didn't expect much, their loss. Soory for the rant...
I do have an SCT Livewire tuner for the 10, but I don't think it covers the 11-12 5.0.... I better make sure before I let the tuner go!