For the mathematically challenged, and working with a tank capacity of 18 gallons, a drop from 360 to 210 miles per tank translates into a drop of from 20 MPG to 11.67 MPG. That's a pretty big drop (over 40% decrease), and more that I would expect from a change in fuel formulation.
JattGary, did you see a corresponding drop in MPG, and did this come on suddenly or over time? If yes, I think you've got a mechanical problem of some sort. If not, then it may be you did not fill the tank completely, or the sensor in the tank is not reading "full".
As an aside, I often compare the MPG shown on the message center with my own hand calculation (for those of you in public high school and the art history majors, that's dividing the total miles driven for one tank by how much fuel you add into the tank). The numbers always work out to be within 0.5 MPG of each other, so I'm confident the message center computer is fairly accurate, at least on my car.