I am changing my gears in my 1992 firebird to 3:42. Will my speedometer be off? And if so haw do I correct?

 

yes it will be slightly off,u can correct it by buying the correct speedo gear for the gears u are installing,the new gear simply replaces the gear on the back of the vss sensor on the transmission.which can be removed with the trans in the car.its a single 10 or 8mm bolt to unbolt the sensor to get the gear out.
im pretty sure that 92's have 3:23 gears either auto or stick,so u wont see much of a gain going to 3:42's.i would suggest going with 3:73's or even a set of 4:10's

 

yeah, if your car is an auto you have 3.23s, if its a stick, you already have 3.42s. if you do a lot of highway driving, i wouldnt recommend changing the gears or your gas mileage is gonna take a poo.