MPG and adding an extra tank?

My MPG BMW R1200GSA w/ DMC sidecar is all over the place from the low to mid 20's to the low 40's.  Wind/crosswind as well as speed being the biggest variables.  While cruising on the interstate 70 MPH gets much better than 80 MPH. Get on twisty roads with lots of acceleration and deceleration and it drops.  Of course the load weight is a factor also.  My GSA has an 8 gal tank, so I typically don't need to carry additional fuel, but when I did the Continental Divide there was one section in Wyoming where making the distance was sketchy so I added a Rotopax mount.