U/D
 

Can anyone bring me up to date on some of the pro's and con's of the underdrive kit on a 99 gt please. Is it a good buy?? speeda, bbk aluminum?? What can i expect?

not really many cons, unless you want to go with a supercharger down the road, then you will have to put your stock pulleys back on. Most people will recommend the Steeda's. I like mine. Felt a bit of a difference when they were installed. Car pulled a little harder, but nothing big. You can expect 5-10hp.

W/ the underdrive will i need a new belt or will the tensioner take care of the slack?

It slows the accessories down, so some have experienced slightly higher engine temps and also little to no charging of the battery at idle.

I personally don't care for them for those 2 reasons and don't feel it's worth the 6-8 hp. I think there other mods that can give you more bang for the buck.

Many on here have had great experiences with them.
If you do decide to get them make sure they are not the piggy-back type.

piggy-back type??

Bolts onto stock crank rather then replacing it. I have Steeda ones and there nice, no charging problem with my car and no heating problems. Power seemed a bit better after on. For the low price i paid for them used it was worth it to me. Steeda ones arent piggy back they are full replacements.

The stock belt tensioner will take care of the slack.

the interior lights are dimmer thats what i noticed when i put some in. i had steedas on my car sold them for about what i paid for em.