Gears
 

What gears would you recommend for my 1999 V6 automatic? Maybe 4.10? And how much of a gain in hp would i see?

Don't force me to become one of thoes people who just says "search", but that is what you need to do to learn about gears. Everything about gears has been covered before, and if you can't find it here I recommend 3.8mustang.com/forum. But I recommend 4:10 gears, t-lok, and a tuner all at the same time. You will see no increase in horsepower just from gears, that is not what gears do. You would get an HP increase from the tuner though. You will see a big difference in acceleration.

ditto.

Horsepower is generated by the engine. Gears only multiply the speed at which the driveshaft is spinning. w/ 4.10s you will see an increase in acceleration but a lower top speed. Also, getting gears is fairly expensive if your on a budget like me. If your going to do it, do it right for a v6. Upgrade the rear end to an 8.8 get a T-lok and you will need to purchase a tuner because your speedometer will be off. Keep in mind that it will drop your mpg's too.

if anything gears are said to hide HP on the dyno.

4.10's-auto

3.73's-manual