That has been my experience also, except that most of mine don't
indicate full or empty accurately!  My Chevy pickup will run 100 miles
before the needle drops down to the full mark and my Sonata will run 50
miles with the needle above full.  My truck is highly nonlinear.  100
miles to get down to the full mark, 270 is at 1/2 and then by 370 it is
dead along-side the road out of full.  Once below 1/2 the needle drops
like a rock and when it hits E, it means it (this is one exception to
empty being inaccurate).

My minivans have been pretty accurate at full, but they will run 80+
miles after the needle hits E.  I've never run one dry so I can't say
exactly.  I have run my pickup dry twice as has my wife once so we know
that when it hits E it means it!

My Sonata is pretty linear throughout the range other than the very top
and very bottom.  It runs about 50 miles down to E, 120-125 to 3/4, 250
to 1/2, 375 to 1/4 and 450+ at E.  I've not run it long on E and never
have run it dry so I don't know how accurate empty is.  When it hits E,
however, it only takes about 15.5 gallons which says it should have 2+
gallons left if the book capacity is correct.

Matt

And just to add to what HT and Mike have provided here, as an Entourage
owner, the gauge is no different than any other current production car.  
It is not accurate.  However, as HT mentioned, I get almost exactly the
same amount of miles out of every tank.  If you watch (and reset) your
trip odometer, you'll always have a fairly accurate way of knowing how
much distance you have left to go.

Removing the middle seats is an excellent idea for the wheelchair.  
There will be some excellent tie-down locations with those seats out.  
One suggestion would be to make sure that you DON'T get the DVD
entertainment system, as that would severely cut the head room in that
portion of the van.  Or at the very least, make sure you or your
passenger tests it out before you buy one with the entertainment system.

Eric