good thread on the subject here:
dr650 fuel mileage Thumpers
Basically, "Your actual mileage will vary"
. The general concensus is that the DR will get between 20MPG and 60MPG LOL. that's not at all surprising though considering that it is a relatively large single cylinder, no aerodynamics, with incredible variation in the amount of drag and load applied. Ever accessory adds drag and weight, riders add drag and weight, knobbies add drag and friction induced drag. A, relatively, low horsepower motor will have large swings in efficiency as it deals with load changes. Also it will be far more susceptible to things like bad jetting (or more likely incorrect for the exact current circumstances jetting), gearing changes, etc. I should clarify that my "30-50mph" efficiency comment was steady state throttle, stop and go mileage will be much worse because the engine is never running at peak efficiency while also travelling minimal distance. Internal combustion engines are at maximum efficiency at full load, in steady state travel that tends to equate to velocity which gives higher mileage (the sum of efficiencies exceeds the sum of deficiencies). There will be a velocity though where the aerodynamic and friction loss drag increases to a point where overal efficiency begin to degrade - resulting in ever decreasing mileage. Thus, best mileage is when the engine is operating closest to maximum efficiency, engine work product (horsepower produced) is used for forward velocity (versus "power" to climb obstacles at lower speeds), and all drag forces are minimal. Anything that reduces engine efficiency (for example not running at the correct stoichiometric ratio), reduces velocity per RPM (shorter gearing), or increases negative forces (friction and drag), will result in lower MPG.
If higher mileage is a goal for you I would start with taller gearing (sprocket change), trying different jetting needles, and looking for friction. Carbs are awesome except when they aren't, worth you taking them apart and rejetting them for YOU'RE desired results if they aren't right.