Common carrier vehicle substitutions

The amount of the registration or license tax applied to a vehicle classified as a common carrier in excess of the amount which would be applied to it if it were not so classified, may be deducted from the registration or license tax due on any vehicle substituted therefor in such service during any registration period proportionately when the vehicle is sold or otherwise disposed of during such period, or is definitely withdrawn from such use or service, and such other vehicle substituted therefor.