question about switching gears?

my opinion is for a street driven car with auto a 3.91 will burn the tires off the thing and still be able to run 80 on the hiway with out any problem.Never did like 3:23 or 3:55 for a performance car gearing ...more like something a salesman would want ....LOL Gas mileage will drop about 2-4 miles agallon (depending on how hard you drive it ). It will cost you more to convert the 9" Ferd and set up the gears than to buy the gears for the 8 3/4 and paying someone to set them up for you (assuming you already have the 83/4 )