Gas Light On Way Too Early

The gas gauge on our 2004 Nissan Armada comes on too early to be useful. We've got a 28 gallon tank, but if you fill up right after the warning light comes on, it only takes 20 gallons. That means we've got potentially another 100 miles before we run dry. So we end up ignoring the warning light and just trying to guess how many more trips to the grocery store and church we can make before hitting the gas station.