I'm curious about the rack your buddy makes. Do you have a pic?
My 95 KDX gets a pretty steady 35mpg. I have the Clarke tank. I forget what they advertise it as but I drained and measured it at 3.5 gallons.
I also weighed it... 249 with a fairly full tank and no tools or extras except armor.
I have a KLR also...
...but the only time I ride it since I got the KDX together is to the store.
Me and a buddy were switching back and forth between the KDX and his WR 426. Even though they weigh about the same the KDX feels way lighter and much easier to ride, especially in tight soft desert washes and winding single track.
After muscling the KLR around for a couple years I went to lift the KDX onto a stand and my buddy said I had the whole bike completely off the ground