So far, we're doing about 640 km between refills, taking about 10 gallons, with maybe 40 or 50 miles to spare before going to reserve. Just got the car a couple weeks ago: 2 tankfuls.
First real test will be a trip from here (Charlottesville, Va) to Columbus, OH area where our daughter lives. The ole VW TDI always did it with plenty of fuel left over, but it has a fair bit larger tank. Also, it never recognized those big W. Va mountains. They weren't there as far as any gear or speed change needed. It'd do an indicated 44-45 mpg, but actual mileage was more like about 41 (the computer lies, just like VW did on those NOx tests). The computer value on the Mazda3 is extremely pessimistic.
The natural gardener