The tank is 2.96 gallons. Also, don't assume that your tank was full when you picked it up, even though the display might have shown that it was.
The best way to get your average fuel economy is to start with a known full tank of gas and a trip odometer reset to zero. The next time you fill up, divide the miles driven by the amount of fuel it took to fill it up, keeping in mind that even that amount of fuel is a variable, in that your bike might be sitting at a slightly different angle than the last time you filled up, nozzle differences at different gas stations can cause the pump to shut off earlier, etc.
Another option is to keep a running tally going on the 2nd trip odometer... don't reset it after each fill up... let it go for 4 or 5 fill ups (though you need to keep track of the amount of gas from each fill up), then you'll have a larger "sample" size of miles driven vs fuel consumed for a more accurate average.