i have a 1998 millenia it will not let me put more than 8 gallons of gas does anyone know what is wrong ?

 

Here is a link to a Mazda TSB that relates very closely to what you are experiencing but would make darn sure that was the problem as the listed combination meter retails for $835!! There are only three left in US warehouses though.

http://www.mstore2000.com/PDF_Files/00-09-003r1-878.pdf

Would want ot make sure that the fuel level sending unit in the tank is functioning correctly and that the tank is not partially collapsed and 8 gallons is all that will fit. Are you refuelling shortly after the low fuel light is coming on? There should still be 2.1-2.6 gallons of fuel left after the light comes on. Were it my car, I would throw a can of gas in the trunk, note the mileage when light came on and drive untill you run out of fuel. If you only made it another 50 miles or so then would point to collapsed tank. If were still able to drive an additional 150 miles or more then would point more to an electrical related problem in sending unit or that combination meter.
You could also just disconnect the fuel line after light come on. Run the fuel pump and measure how much fuel is left.