I have to agree with those that go for carrying jerry cans. It's much more cost effective.

On the subject of filling up I usually do it when 1/4 tank is left just so I can get out. In fact I usually watch the time and stop every 1.2 to 2 hours on road trips. The drive is always the best part; if you stop every 2 hours there's a better chance of wanting to stop at the "Biggest Ball of Tin Foil" or whatever the local claim to fame is.