Where can i get my gears changed and how much would it cost? 3.8 v6 2002 mustang?
You can have it done at any good shop. It will raise your rpm's, which will definitely make it feel quicker. It won't add any horsepower. The down side is on the highway, you'll be driving along at about 2500 rpm's. This lowers your fuel economy.
While you're back there, have them add traction lock so the power goes to both rear wheels instead of just one. GT's and Cobras have it. That way there's no more one-wheel burnouts.
Cost for all parts is probably about $400 to $500, labor depends on the shop. That includes traction lock.