when upgrading a 99-04 v6 mustang's rear end from the stock 3.27 gear ratio to 3.73 gear ratio you stang to shave off at most 2.5-3.0 tenths from your times. This is well worth the efford since most other bolt ons are lucky to net you a single tenth.
It is far better to buy and install a ford traction lock with the gears since the exact same amont of work goes into installing gears as installing a traction lock and gears. So that being dont let someone charge you double... basically when they sit the stock open differential down on the ground or tray to pick up the ring gear you slide the tlok diff in its place and thats really the only extra effort it takes.
www.gefracing.com has really good prices, make sure they are not out of stock, if they are or you just want to you can go to summit and order the stuff and they will price match with gefracing.com and may even give free shipping... so go that route.
It is a better idea to upgrade to an 8.8 rear end from a GT mustang (99-04 gt mustang 8.8 rear end is required UNLESS you dont mind running wheel spacers with a pre 99 8.8 rear end)
if you dont mind wheel spacers (
www.maximummotorsports.com has very good wheel spacers!) then go to
www.speedconcepts.net and buy a comoplete ready to bolt up 8.8 rear end (minus axles which you can use from the stock 7.5 oem v6 rear end perfectly fine, they are the same ones used in the gt rear ends. )
the swap of axles is very easy and is all that is needed to put the 8.8 from speedconcepts.net into your car slap on the C clips from the 7.5 oem v6 rear end and the spacers and wheels and your done minus filling it with fluid and breaking them in.
If i had to do it over again I would go with an 8.8 in a SECOND, I have broke my rear once... I dont plan to again. The gears wont break .... the spider gears from the tlok unit or open differential will....
the 8.8 from speedconcepts is 725.00 ... which is about what you are looking to pay for 3.73 gears, tlok unit, and install prices. the 8.8 comes with 3.73 gears and a tlok and as listed above all you need is your stock axles and thats it.