law: 7. Liability for additional tax.-
  When any motor vehicle in respect of which a tax for any period is payable or has been paid, is altered during such period, or proposed to be used during such period in such manner as to cause the vehicle to become a vehicle in respect of which a higher rate of tax is payable, the registered owner or the person who has possession or control of the vehicle, shall, in addition to the tax, if any, due from him for that period, be liable to pay for the unexpired portion of such period since the vehicle is altered or proposed to be used, an additional tax of a sum equal to the difference between the amount of tax payable for such unexpired portion at the higher rate and the rate at which the tax was payable or paid before the alteration or use of the vehicle for that portion; and until such additional tax has been paid, the taxation authority shall not grant a fresh token in respect of the vehicle so altered or proposed to be so used.
  Explanation-In calculating the unexpired portion under this section any broken period in a month shall be considered as a full month.