5th. is mainle for for Ist. use. The other 4 are same as 4spd.
If most of your driving is city 5th is not much use.
For racing 4th. is good for 90+, very little gain in 5th.
I you have a daily use at highway spds.[ 55+ ] a 5spd. will help gas milage, but it will take a lot of gas saving [ 2 to 3 mi/gl. at 55+ ] to pay for the change. Just my .02