The gauge doesn't work at all, but it will run badly for a few miles before I get to a point where I don't get enough gas to move.

 

Clean out your gas tank. That's the first thing i'd do. If it doesn't change, check into your fuel lines etc. If it's coolant being burned it'll be white all the time and smeel somewhat like cat ****. Then you're likely to need new head gaskets. Happened to a 98 1 ton we used to have. It had a 454 and burned a LOT of coolant. The exhaust was white and it reeked, even with the windows open on the highway.

 

it sounds like a coolant leak in the head. my moms windstar had a head leak and blew white smoke, occasionally leaving a trail about a hundred feet long. go to napa and ask for coolant stop leak. it mixes in with the coolant and fills any leaks.

 

Coolant stop leak is evil. It DOES stop small leaks, yes, but if there are any small passages in the engine, it can also clog them up. A friend of mine had ended up having to pull the head on his MR2 because he used a bottle of stop leak and it clogged a coolant passage on one of the cylinders, and we ended up with nasty detonation as a result.

 

It may just be water in your fuel.