I've have a 2005 GMC 2500HD Duramax. 3inch suspension lift. Bully dog tune. Bush whacker 3inch fender flares. Sitting on Cooper 33x12.5 New on the diesel scene. Looking for general tricks and knowledge. What are some good investments/ upgrades. Appreciate any reply.

 

welcome to db.
you can also post your other question in the duramax section for a better response.

 

Free up the air flow. Be careful not to screw with the maf sensor.
Is yours the LLY or LB7 I' m not sure when they changed.
Check your emissions package its on the code sticker in the glove box. EF9 on my 04.5 is mod friendly.
If you have any modifications post them in your signature.

 

My bad LBZ not LB7