'89-'91 tanks are just over 19.0 gallons...Not sure of the exact volume.
'92-'97 tanks are 18.8 gallons due to a change in the exhaust system.
typical mileage can range from 22 highway to 26 highway miles per gallon. In and around town mileage varies greatly depending on driving habits...I don't get very good mileage around town!
Modifications also play a role in that. Although highway miles are pretty constant, around town mileage goes down with the more mods you have...It's all that acceleration...