You want to find out the approximate number of miles you can ride before you're dry, right? I think this one has been covered, but here it goes:
1. Find out how large is your gas tank (call this T). Jax184 said it was 0.79 gallons but you can verify.
2. Fill up your gas tank to the max. Write down your mileage (call this M1) and keep this safe.
3. Ride for a day or two.
4. Go back and refill your tank to the max. Note how much gas you need to add (call this G1) , and write down your mileage (call this M2) a second time.
Your miles per gallon MPG = (M2-M1)/G1. How long you can theoretically ride D=T*MPG. If you keep a log of when you gas up over a longer period of time you'll get more calculations andtherefore more accuracy. Gas leaks from the tank/petcock invalidate your calculation.
Give yourself less mileage before you fill up, because riding against the wind, riding in cold weather or 2 up riding will give you worse mileage. For example if D=150 miles/kms, then only ride 125 miles/kms before you tank up.