I'm hoping someone can give me some real world numbers of HP and Torque gains with different mods. (I know it depends on the car, but I just don't trust the advertising hype) I'm trying to decide if these mods are worth the trouble.
Underdrive pulleys?
Exhaust?(headers,X-pipe...)
I have a K&N filter, and have removed the air silencer. How much of a gain is that good for?
Also, how bad a hit am I taking on gas mileage if I go to 3.73 gears. (obviously I didn't buy a Cobra for its gas mileage

, but I would like to know.)