Gear

Check this out...

http://www.edbert.net/rpm_chart.htm

Use the "3rd" gear portion for C4, the "4th" is for the AOD (duh!).

Your sig tells very little about your engine combo.
It doesn't make much difference since those gears are so close, but I could give reasons one way or another if I knew more about your cam and intake.
As it stands, I lean 3.80.
Reason being that the auto tranny has a fairly high first gear ratio. If you had a manual tranny I would lean to the 3.55.
Either gear you choose will seem like a huge difference from what you have now. If you switched a 3.55 for a 3.80, or vice-versa, you would probably not see a noticeable difference.
I did work under the assumption that your car is a small block, then judging weight, tire, intended use, and tranny... I like the 3.80.
Dave

3.73 - 3.80 is fine for your situation.

HistoricMustang
www.historicmustang.com