zillow is definately out of wack on the high side.
tax assessor, by law in most states, are actual FMV according to the figures they have available to them. Recent sales (comps) within the county, but they can not "discriminate" in any way. So I have found that their comps are any home of the same general size, shape, and condition not matter where it is in the county.
This tends to make them error on the low side of what most would consider FMV and most home owners attempt to get their assessment lowered by any way they can, legally, so their taxes are less. Also you may find that the assessors figures are often 1-5 years behind, depending on how long it takes to get around to doing at least a drive by of each house in the county. This also means that they will miss some of the "improvements" that actually add value to the FMV and the homeowners wont report it (if not required to) to keep their taxes lower.
Therefore I would use it when purchasing a home to bring down the FMV in my favor as a buyer and Zillow to bring up my FMV if I am a seller.