Here's a hint- inside the stock fuel tank is a off clear sleave in the top of inside where the cap fits.When you fill your tank it leaves the top 1.5 in. empty -that thing comes out. Most guys fill it up, then they think its full. There is a small hole in the side of the sleave that lets it even out,but by removing it you get a true full tank.
Now you might be thinking why is it in there? Well I can think of maybe two reasons 1{exspantion}-you have a air tub to help with that problem 2{easier not to spill gas}-well my take on that is watch what your doing.
ok if you can think of any more reasons let me know please.
To remove the sleave I took a sloted screw driver very carefully prying inword where they slid together then when you have room to get a pair of needle nose pliers then pull strieght out the spout don't damage the area where the cap treads on.
My buddy rides a 2000 xr650r he told me about it ,his is the same.We ride as far as we can so an every little bit of fuel might keep us from walking, we both have 32 oz.fuel bottles straped to are bikes an I haul a 3 gal. can on my six pack rack for 100-150 mile days