How far can I go after putting the fuel petcock on "reserve"
You'll usually get 35 to 50 miles on reserve. There are about four gallons available in the main part of the tank and .8 in reserve.
Reserve should be used frequently. When the petcock is switched from main to reserve, the fuel that goes to the engine comes from a lower part of the tank, and the lowest part of the tank is where the water and dirt go if left alone. If the reserve is never used, the level of the water and dirt in the bottom of the tank continually rises. If it gets above the level of the reserve fuel line, it goes into the carbs when the petcock is switched.
Most riders prefer to have a little water and dirt go into the carbs every tank of fuel, rather than a lot of it at a time when they're already in trouble because the fuel level got a lot lower than normal.
This is one of those messages that needs a disclaimer. The correct method is to remove the tank, remove the petcock, then completely drain and rinse the tank. This should be done more frequently if the bike is stored outside where it gets rained on. Think for yourself and do what you think is best. Dispose of contaminated fuel in accordance with local laws.
There exists a trick which allows you to squeeze a few more miles out of a seemingly empty tank. When you run out of reserve fuel, there is still a little gas left! This is because most motorcycle tanks are shaped like an inverted "U".
When the fuel level gets too low for even the reserve intake, there is still a little remaining on the other side of the tank. If you lean your bike to the left (to be more correct: to the petcock side) very far (cautiously, so as not to drop your bike!), then the remaining fuel will flow to the petcock side and you will be able to ride a few more miles.
This is what the petcock looks like when taken out of the tank. Notice the long and short 'straws':