o the ones i grabed were about 18 inches. the deep er the sound the happier i am

 

If you went with a 36" glass pack it would be quieter. Eventually though, glasspacks burnout and turn into straight pipes.

 

Ill put my opinion in here. I ran a Thrush 3in welded muffler (knock off flowmaster) cost like 29 bucks at my local autovalue. Went to a 36 inch glasspack sounded like crap and resonated in the cab horrible. Went back to my welded cheapy. I put one on both of my trucks

 

so im now runnin strights and telling the cops to look up the law on old diesels. (no emissions)!!!

 

muffler has nothing to do with emmisions. here in pa if it came with a muffler it is needed. but by pa law if the truck has a turbo it does not need a muffler due to Mack calling thier turbos noise suppressing deivces. but a nonturbo still needs muffler. but thats here in pa.

 

here in tn noise is up to each cops view, but i cut off a set of flowmasters to run straight pipe. if you run brand new glasspacks and get them hot then spray water inside them they will bust and you get the sound of straights with the look of muffers for cops.