Gears are great but the cost on a 99+ car can be really expensive when you consider the cost of gears , tlok and a chip to correct the speedo. It could easily cost over 800 dollars to do the complete gear change right.
The underdrive pulley, CAI, If the car is a few years old give it a tune up and clean the MAF with non residue electric contact cleaner. This would also be a great time to maybe even get a shift kit
Well if you have not heard of this before I am surprised. But here is the low down. If you have a 99+ mustang and get 3.73 or 4.10 gears the speedo can only be corrected by either a Chip or Speed cal device. On Mustangs up to 98 you only needed to change a tiny plastic speedo gear. On 99+ the computer controls the speed corrections
If you need anymore info please feel free to ask. I will not steer you wrong