You can get a very good idea what gears you have by jacking up the rear end and rotating the tire while counting the revolutions of the pinion (I am sure there is a YouTube video showing it).
I don't remember what the standard gears were in the TA but if I had to guess I'd say 3.23 or 3.55. It is very possible that someone put some 2.76 gears in out of something else at some point.
You have to look at the gear ratio as a part of your overall drive train (engine, trans, tires). Deeper gears like 3.55, 3.91, 4.10 are good for the low end but the deeper/lower gear you go the less friendly it will be on the highway. This is one of the reasons people like overdrives, 5 or 6 speed trans, etc. because they can have a low gear for ripping around but then use the overdrive (or 5th-6th gear) to cruise on the hwy.
As stated, your engine and trans play a big part in this situation, if the engine is stock with a manual trans than you might want 3.55 gears. However if it is stock with an AT you might 3.23 gears. The 3.55 will help the car get off the line with the manual without you having to rev the crap out of it. Now, if you are making good power then you can probably go 3.23 with a manual as well as a AT and be good.
To me 3.91 gears are a bit steep for the hwy but I am sure there are those that disagree. Anything deeper than that IMO is going to howl on the hwy and require some significant RPM to maintain speed.
Tire size plays a part in this equation, if the tires are tall then it is like having tall gears (3.23, 2.76, etc.) whereas if they are smaller they act as if you have a lower gear. Point here is that many people have their car all set up and working the way they like it and then put taller tires on and wonder why the car is slower to get going.
You need to figure out what you have first and then you can make a decision on which way to go.
