The amount of tax you owe each year is based on the value of the vehicles you owned on January 1st of that year. The value of the vehicles is established by the County Assessor using a standard rate book. The assessed value of your vehicles can be found in the upper center portion of your statement. Your bill is also determined by the city in which you reside. Your tax bill may appear higher this year because city and county taxes are now on the same bill.