21st Century Transportation Fuels Act This bill addresses provisions related to high-octane fuel, renewable fuels, and vehicle fuel efficiency. Among other things, the bill establishes a national octane standard that requires automobile manufacturers to design motor vehicles to operate using gasoline that has a research octane number of 95 or higher, addresses the limitations on Reid vapor pressure (a measure of gasoline's volatility) that are placed on gasoline during the summer ozone season, and revises requirements for calculating Corporate Average Fuel Economy (CAFE) credits earned by a passenger automobile manufacturer for exceeding minimal required CAFE standards in a particular model year.