US gasoline prices have dropped to their lowest levels in 4 years, and they are surely going to go lower as oil prices plunge. The national average price for a gallon of gas is now $1.997. In few locations it is even selling less than $1. In Hawaii and California, however, the average price is still above $3. But drivers are paying less than $1.99 at more than two-thirds of the stations in the US.