When I changed the rear gears in one of my toy's from 3.23:1 I thought that the 3.56:1 would not be enough of a difference so I went with 3.92:1 gears.
Biggest mistake that I ever made! the car is .02 seconds faster in the 1/8 mile and my gas mileage went from 10.5 mpg to 6.5 mpg. Now it is a pain to take on the highway and just sucks more gas.
The 3.55:1 in my Saleen, is just about ideal with the fifth gear O.D.
I would not go higher than a 3.73:1 in your car. Just not worth it.
Unless you were to install the 6-speed from the shelby's with two O.D gears.

Eric

Might want to just save your $ right now. The tax man cometh.

I believe it would be more of a waste to go from 3.55s to 3.73. The 5 speeds in the S197 has almost the final gearing as the 6speed that is offered in the GT500. It seem when you changed gearing from 3.23 to 3.56 your MPG dropped by around 40%. I guarantee, if the OP keeps the same driving habits his MPG will not drop by 40%. I can almost guarantee that is overall MPG will stay within 1-3 MPG.

My Saleen has 400 RWHP and the 3.55:1 is sweet. Third gear at 70 ish scoots right up to 95 plenty fast, uphill. And it still gets great MPG. Very nice set-up.
No problem at all passing everything on the road. I am going to install a smaller pulley on the blower this week and it will only get better.

Eric

You have a manual and if you want to compromise get the 3.90's. make sure you have an installer that knows what they re doing to and give a guarantee on "No Whine!"

If going with a twin screw S/C, then 4.10s might be too much, although one of my friends is running a KB on 05 Mustang GT with auto and 4.10. The car is his d/d. Only problem is, that he has to use drag radials for every day driving.
3.90s might be good, but I have not read one post were some one with 3.90s didn't whine. It didn't mater what company they came from or who did the install.

4.10s!! Everyone who has them loves em. Highway rpms are not that much of a difference, and ur mileage in city will increase without much lost on the highway as long as you don't drive 80+ everywhere. Don't waste your time with 3.73s

LOL!! never would have guessed it Joe

You'll get a million opinions on this one. Some people with a twin screw say 3.73. I know others (Kolb02 for instance) who highly recommend 4.10 no matter.

Honestly if you can get traction then 4.10 would be my choice no matter what the situation.

40% ????? Don't think so!

If he's getting 20mpg now, he's not going to get 12mpg after getting 4.10's.

I get about 17-18mpg with my 4.10's. Closer to 20mpg if all I do is highway driving.