Benson, you will find the information you're looking for at
www.ASTM.org. This is the American Society for Testing and Materials and is responsible for writing and maintaining the specification for gasoline in the U.S. It also rides herd over diesel, jet, and aviation gasoline as well as other fuels. The spec you want is ASTM D 4814, Standard Specification for Automotive Spark-Ignition Engine Fuel, and can be found in Volume 05.02 of the Pertroleum Products and Lubricants manual. There are tables that indicate when and where the volatility (vapor pressure and distillation) changes and also the T20 V/L. MrTexas has it right about the numbers; the tables set the parameters above on a monthly basis although certain areas don't change much (if at all) from month to month. ASTM will charge you for the spec if you're not a member; PM me with your area and I can give you the numbers you need.