2000 Gt Mods

you will get a lil worse gas milage the higher you go...basically think of it this way..with higher gears at the same speed you will be at higher rpms than with lower gears
 
Your gas milage probably wont change that much. After my 3.73's I filled her up a few times and got 290+ miles out of the first tank, then got 320+ plus miles out of the 2nd tank. This is not being easy on the car either. This is about 50/50 hw/city driving.
Also, WELCOME :D
As for your questions, Id get gears, a chip, and pulley's. You wont need a T/A as long as you get a chip to recalibrate your speedo when you put in your new gear ratio.
:nice:

Best of luck!