I almost always wait until my miles-to-empty is at or near zero. And when I fill up, I almost always put between 13.6-13.9 gallons in. I would say that about 75% of my fillups fall into this range. I've only had the NAV run high 5 times and be on par with the manual number about twice. I don't think that leaves much room for error over the course of 30

-Tim

2006 Mercury Mariner Hybrid AWD
Black with Pebble interior
Premium Package with Nav & Moonroof



Current ODO: 26,152
Typical Drive: 20 min crosstown in heavy traffic (3.5mi there and back twice a day)


532 Gallons of gas saved
That's 10,642 lbs less CO2 emitted