You have to assess your use of the car in primary (most common) driving style. Example, with a 3.31 rear end, 50 aspect tires and 15" wheels with a close ratio trannie (I have a 2.32 Toploader), you may NEVER get into third gear until 60 MPH or so. RPMs at 30 MPH in 3rd would be a boggy 1600 and 40 MPH would be 2100.
Meanwhile, if you had a 4.11 rear with the same close ratio trannie, 30 MPH rpms in 3rd would be 2300, and 40 MPH in3rd would result in 3100 RPM. If you know your engines torque curve you can then figure out which trannie and rear end combo would get you into a sweet spot for your primary driving style.
Here in CT, I do alot of cruising on back country tree lined hilly and curvey roads and having my RPMs UP is nice and in the sweet spot of my torque (2800 to 5600) where it is flat at 435 to 485 Lb-Ft range .
Also, when you step on the gas with a 4.11 rear ratio, you just are subserviant to the massive G force. A true rush and a half. Another benefit is you get to shift up and down more. I suspect with the 3.31 rear end I would virtually never get into 3rd gear in the city or below 50 MPH.
You are probably wondering how I got that data??
There is a great program you can download called RPM.
It is located at
www.gearzone.net and was written by Paul Blaccard. It is a really nice tool. He also has a great article that explains wide and close gear ratios in Trannies....
http://www.5speeds.com/ratios.html
You get to enter in your trannie, its gear ratios, Wheel size, tire aspect, tire width, rear end ratio and then by choosing a speed you see the RPMs you should theoretically experience in each gear. You can compare two different sets of data as well.
One tip, I am running 305-50-15 tires on the rear of ERA375. The program tends to not work as well with the wide tires. So I found that if I chose 205-50-15 then the results were just dandy.
I made a nice Excel spreadsheet to store the data.
Gears across the top and MPH down the side and the intersecting cells have the expected RPM.
Fun.