My 84, with  5 sp manual and an I-4 motor conversion, I go routinely 300 and then start looking.  I have gone 340 and it took 13.2 gallons from there..  That is about as daring as I have been, though if indeed the tank holds 15.9 gallons, I should be able to get a safe 375 miles..using my new figures after doing some engine work I seem to be getting about 23.5 mpg in 'mixed driving'..Probably again be about 25mpg on highway trips..

  My fuel gauge is way odd..it never reads all full and within about 50 miles its nearing 1/2 full..When I get onto the red, it means I have about 150 more miles of range..Good thing my trip odo works.  The gauge also moves it's reading depending on the Van's attitude...

  If you are curious, you might carry around a couple a gallons and run it right out...I'd bet you wouldn't hurt anything if you stopped as soon as it started making noise at the fuel pump or the engine starts to sputter..Put the fuel in and then go change your filter...You would know exactly where empty was then..
  Don Hanson