Any toilet sold since 1992 uses 1.6 gallons or less.
1.6 gallons unless someone has changed out the flapper during that time, which would mean that you are draining the entire tank instead of letting out the 1.6 it was designed for. If you are draining the tank, then you're using 2.5 or more.
Can you attach a picture of what you have?
New water saving toilets have the Watersense label. Watersense is 1.28 gallons or less.