(a) An excise tax is hereby imposed for the use of fuel at the following rate per gallon:

1. (1) Fourteen cents ($0.14) during 1990, on and after August 1.
2. (2) Fifteen cents ($0.15) during 1991.
3. (3) Sixteen cents ($0.16) during 1992.
4. (4) Seventeen cents ($0.17) during 1993.
5. (5) Eighteen cents ($0.18) on and after January 1, 1994.

(b) If the federal fuel tax is reduced below the rate of fifteen cents ($0.15) per gallon and federal financial allocations to this state for highway and exclusive public mass transit guideway purposes are reduced or eliminated correspondingly, the tax rate imposed by this section, on and after the date of the reduction, shall be increased by an amount so that the combined state and federal tax rate per gallon equals the following:

1. (1) Twenty-nine cents ($0.29) during 1990, on and after August 1.
2. (2) Thirty cents ($0.30) during 1991.
3. (3) Thirty-one cents ($0.31) during 1992.
4. (4) Thirty-two cents ($0.32) during 1993.
5. (5) Thirty-three cents ($0.33) on and after January 1, 1994.