I have a 2003 and it's getting about 100 miles per tank...about 45-50 mpg highway.  I was wondering if anyone knew how i could improve my mpg.  I have installed a K&N filter in air box.  I am going to install a dynojet stage 1 kit (should be here next week) and I want to make the stock pipe louder like a thumper not a chirper!  
thanks for the info

Installing the K&N and the DynoJet stage 1 kit will make your fuel economy go down, not up.

If you are looking for maximum fuel economy, put in a smaller idle jet, a smaller main jet, replace the white spacer with 3 washers (approx original thickness) and turn your A/F mix to 1 1/2 turns out.

The bike will run like crap and backfire alot, but you will be tuned for maximum mileage.

Going the route you are, you are tuning for maximum power.  It will only go down the more things you do.

I'll be interested to know what you find when you take your carburetor apart. I'm curious to know what would allow your motorcycle to run at speed for ten miles and then cause grief.

Last year I took a little trip on my '07 S40. On one leg I drove at 70-75 on the Interstate, and got about 39 mpg. Most of the time I run it at 60-65, and get mileage in the low to mid-50s. On one trip I drove around the Rocky Mountain Park and environs, at their low speed limits (45 max, some slower) and got 70+ mpg. Of late I have been running around 55-60 and getting mileage in the high 50s.

Except for removing the idle screw cover and adjusting the idle mix, my S40 is box stock. Contrary to Skrapiron's experience, my bike runs just fine, and I get only occasional backfires on deceleration. I tried the "white spacer mod", though I didn't take it to completion. I removed the spacer and installed 2 4-40 washers (the spacer is about 3 washers thick), and fuel mileage dropped immediately from low to mid 50s into the high 40s. I could discern no difference in performance, and the backfire didn't go away. I replaced the white spacer and my mileage went back up.