It seems that My MilleniaS doesn't have a 18 gallon tank

What is your question?

My Millenia is a salvage that I bought with 19000 miles. Salvage was for body damage only. This car has been great. The only problem I have had is the thermstat stuck which I changed for $11. But one problem that I can't figure out is that when I fill it up the most it has ever taken is 14 gallons. That was when it was on fumes too. Have you heard of this before. I realy need that extra fuel range.