[Eric K]

A few observations on increasing fuel efficiency...

1) Float and Jetting – this is the first step. Make sure the float is set correctly. Dial your jetting in so the plug looks good, the engine runs smoothly across the range with good power and throttle response. The tail pipe should have little oily spooge when you are jetted well. The CCK needle is one of the best in my experience for performance and fuel mileage.
2) RB designs head mod. This will reduce the amount of unburned fuel and improve power (primarily low to mid) by improving combustion efficiency.
3) down hills – the PWK carbs will drain fuel out the overflow when you are descending hills steeper than about 30 degrees. I first noticed this when following my oldest son down hills. I set the motorcycle up suspended at a downhill angle and found when the fork tubes were near vertical the carb would drain lots of fuel out the overflow. This is with the float set to spec. This becomes noticeable on long (1/2 mile to 1 mile) steep descents. Minimize your time on the descents and/or shut off the fuel and down hill mountain bike the ride.
4) riding style – maintaining corner speed with a smooth flowing style notably improves efficiency.
5) [for Rekluse users] proper Rekluse set up with TC balls and a low RPM engage point to avoid a lot of low speed slip

On technical trails (like these -http://www.fs.fed.us/gpnf/recreation/trailbikes/trails.shtml ) I get 30+ mpg riding alone. I ride to maximize cornering speed. I weigh about 270 lb geared up.

I get about 22-25 mpg on the same trails giving my 6 year old son a ride. With him aboard most of the riding is 1st to 3rd gear with the majority of the time in 2nd gear. He weighs about 95 lb geared up. We corner slower and use more fuel accelerating out of corners. There is more clutch slip on uphills giving him a ride. There is also more fuel used due to the added mass. This fuel mileage includes multiple 3-4 mile long climbs from under 2000 ft elevation to nearly 6000 ft elevation.