I have an 07 gt, and debated this gear thing as well. I had a 98 cobra that I had 4.10's installed on, and I absolutely loved them. I had several upgrades, and it was a beast, so gas mileage was never a big concern, as I only drove it to the track or on weekend night cruising. But as for the 07 gt, I chose the 3.90's. I got the pro 5.0 gears, and the rpm is around 2100, 2150 at 60 mph. They are way better than the stock 3.08's or the standard 3.55's and they are a great all around gear for these cars. Not quite as fast as the 4.10's or 4.30's, but with the right upgrades, and or power adders, i.e. blowers or nitrous, or a turbo, you would be very happy with these. They are just a good all around gear.  

Unless you do alot of highway driving you will LOVE the 4:10 gears. They make a nice difference at the strip or roadrace track.

Agree with 3:73 with poweradder.

Many of our customers have done this upgrade, no regrets thus far.  

I have an 05 GT and installed the Ford Racing 4:10's. I drive the car on a daily basis and I still get 27 mpg on the highway. It is great at the track and the street and still maintains reliability.  

I have a 92 lx with 4.10s and a tremec 3550 trans.  At 60mph it reads 2200 on the tac.  I drive 270 miles to work each week.  I plan to switch to 3.73 or 3.55 to save gas and wear and tear on my engine.  The 4.10s are great for red light to red light but for highway use go with 3.73s  

I have a 96' gt with 4:10's works great for a naturally aspirated, supercharged or nitrous engine, fuel mileage didn't decline that much and i drove 100 miles daily (the car had 2.73:1 before)and had gobs of power on the bottom end. Depending on whats in your future for mods you may want to stay with 3.55:1 if you run a turbo you still get great mileage (as long as you keep out of boost) and the lower gears significantly improve turbo spool with the boost coming in from 2500-3500rpm.