Amends the Municipal Motor Fuel Tax Law in the Illinois Municipal Code. Provides that, for purposes of the Law, a municipality is "in a county with a population of over 3,000,000 inhabitants" if any portion of the territory of the municipality is located in such a county. Provides that, notwithstanding any provision of the Law to the contrary, a municipality whose territory lies partially in a county with a population of over 3,000,000 inhabitants and partially outside such a county may, in the alternative, impose the tax authorized under the Law in only that portion of the municipality that lies in a county with a population of over 3,000,000 inhabitants. Effective immediately.