How low do you let your tank get before stopping?

I stop around a quarter, but I have a 42gal tank so that gives me a bit of time to decide were I want to stop as it gets close to that quarter mark. We are often ready to stop by that point anyway.

We usually do long trips with the in-laws (they like to camp the same places we do). They have a much smaller tank in their Tahoe, so we just stop when they do and I never have to worry about how much gas I have!