5 horse gooseneck- 13-14 depending on load, headwind, etc.
2 place snowmobile trailer with sleds, gear usually 17-18...

 

I was haulin a jd 7330 weights right around 13k on a 32ft gooseneck and driving theough town with a 6spd SUCKS!!!! But out on the open road i got 16 but i am in the flatlands of kansas!!!!

 

my duramax will get 16 with a 24' gooseneck stock trailer empty and about 12 when loaded with hay on my 32' flatbed (gcvw about 32,000).
My cummins will get about 14 w/ stock trailer and 11 w/ same load on flatbed, but i've just barely done anything to it yet. It will be considerably better when i get done with it.