Fill your tank
Last week I saw several reports about a sharp jump in people running out of gas on the highway. The stories attributed the increase to high gas prices, suggesting that people who were short of cash simply didn't have enough to fill their tank. That's a good example of how it's very expensive to be poor.
(Two examples: The AAA reports calls from out-of-gas AAA members in Philadelphia have doubled. In Atlanta, the taxpayers will foot the bill for a gallon or two, as Jason White described in One way to combat high prices.)
Notwithstanding a few sources of free gas for people who'd otherwise be stranded on the highway, it's pretty darned expensive and inconvenient to run out of gas. In many big cities it's against the law to run out of gas on the highway--you may get a free gallon of gas, but it comes with a ticket that makes it no bargain at all. Even if you dodge that bullet, your plans are thrown into total disarray--along with those who were counting on you, plus anyone you might call to help you out.
It'd be one thing if buying less gas saved you money. But it doesn't. What saves you money is driving less.
After all, it's however much you drive that determines how much gas you need. If you don't change how much you drive, there's not really anything you can do at the gas station that'll cut down on how much gas you need to buy.
In fact, I'd go on to suggest that it makes good sense to keep your tank reasonably full most of the time. Think of it as an investment--gas in your tank is an asset just like cash in your checking account. It doesn't earn any interest, but the interest rate on most checking accounts these days doesn't keep you even with inflation anyway. As an investment, gas in your tank has done pretty well just lately.
The main win, though, is not the "investment return," but the convenience of hardly ever running out of gas, plus the convenience of not having to worry about minor disruptions in the supply of gasoline.
So, what do you do if you can't afford to fill your tank? If it's just because you're a little short until payday, that's one thing. But if payday comes and you're still a little short, that suggests that you're living beyond your means. (Admittedly, means severely pinched by the recent rise in gas prices.) You might consider driving less. If you can't drive less, you might look over your budget for other possible savings. But your spending on gasoline depends on how much you drive, not how full you fill your tank.