You can search on "miles per gallon", and some other terms, and discover all kinds of threads on what people are getting.
Figure somewhere in the 50-60 mpg for most people, and you're in the ball park.
Since you have a rotopax just fill everything up, reset your tripometer, and ride until you run out of gas.
Then use your rotopax to get yourself to a gas station.
3 gallon tank = 150-180 miles
(maybe more - maybe less depending on your throttle hand)
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs