I ran the IMS 4.0 gallon tank. You have to have either really small hands or don't be afraid to burn yourself if you think your going to adjust the idle with that tank. I could do road side jetting changes without removing the tank, but several times I thought about making some kind of quick release as you have to forget about pulling a plug without pulling the tank.
When I first started the trip I was getting between 55 to 65 MPG on super unleaded in most road/off road conditions, but I noticed depending on the state I was in a 15% to 20% loss of mileage due to older or unregulated ethanol blends. Arizona fuel was absolute crap! In northern AZ going toward New Mexico my mileage dropped off to about 43 MPG. I almost ran out of gas a couple times between stops. Also the 4.0 IMS tank will run out of gas with as much as 2/3rd a gallon still in the tank. I had to stop and lay the bike completely on its side to get the fuel from the right side to the left. That totally sucked with a bike loaded with gear pushing 350lbs.
I imagine if I didn't have all that gear, (I did realize after the trip was over I could have left half of it at home) and I could have carried an extra chain and sprockets for the off road days I did in Moab UT and central Oregon I would have set up a more Hwy friendly 16/38 and then run a 14/44 for off road. I learned the hard way that although my almost stock XR400 was an alright DS at Hwy speeds its really a low speed forest bike. I imagine the new 440 11:1 kit and the stage 2 cam, CRF cam chain and prepped carb will change my thoughts a bit when I finally get to ride her.
I have several other bigger touring and bigger dirt bikes in my collection but I really love my little XR4 and will keep riding her till the wheels fall off.
Good luck with yours.