How much Fuel is enough?

[Desert Runner]

I'm planning a trip through the upper Mid-west this summer (North/South Dakota, then into Wyoming and Colorado). I expect along the highway portions (which honestly will be the majority) that fuel won't be too hard to come by... but when I start adventuring off the highway, how much fuel should I keep available? I figure my main tank (16 gal) will last ~250 miles, give or take.

At least 1,.... 5 gallon spare and your tank as full as possible at the trail head entrance. Doing a winter project(Canada/Alaska), we were instructed to fill the vehicle at every opportunity because many places were either on reduced hours, or closed for the season during the winter.