The supplemental motor vehicle grand list is produced in accordance with §12-71b of the General Statutes. This law provides that a motor vehicle which is not registered on October 1st, but is registered subsequent to that date and prior to August 1st in the assessment year, shall be placed in the supplemental motor vehicle list. The property tax bill for such a vehicle is payable on the first day of January following that assessment year. The supplemental motor vehicle tax is a prorated tax. The listings sent to towns are coded by DMV to indicate the month in which the transaction was validated. The following chart outlines the coding and proration factors for supplemental motor vehicle bills:
Month | Codes | Percent | Month | Codes | Percent |
| October | A & N | 100% | March | F & S | 58.3 |
| November | B & O | 91.7% | April | G & T | 50.0% |
| December | C & P | 83.3% | May | H & U | 41.7% |
| January | D & Q | 75.0% | June | I & V | 33.3% |
| February | E & R | 66.7% | July | J & W | 25.0% |
Newly registered vehicles are the first code, and if it is a replacement vehicle (same plate used) it is the second code.