I have Slash down Jackpot mufflers installed, ( on when bought bike).   If I measure opening end of muffler across, the measurement is 3.5 inches, if I measure top to bottom the measurement is 4 inches.   What size mufflers do I have???  Thanks .   By the way , they sound great. 

you have 3.5 x 4 inch mufflers mate

Jackpot mufflers come in 3.5 inch and 4 inch sizes I've been told. As i can get both dimensions by the way you measure the pipe, are they 3.5 or 4 inch mufflers??  I take its the way you measure, top to bottom , or left to right.    Any ideas.  Thanks

 3 1/2" is what you have.

Sorry I should term that better, your Muffler's are 3.5" in Diameter.