By JONATHAN FAHEYNEW YORK (AP) a¿¿ Local gasoline prices are swinging up and down ever more drastically, a result of a national fuel system that is operating with a shrinking margin for error. Jumps of 20 cents per gallon or more in a single day are becoming more common, for example, according to an AP analysis of daily and weekly price changes at 120,000 U.S. gasoline stations tracked by GasBuddy.com. Sixty-three times this year at least one U.S. metro area has seen such a change. Like the 24-cent increase Decatur, Ill. drivers saw on Jan. 26, or the 24-cent increase in Superior, Wis. on April 30, and the 28-cent increase in Henderson, Ky. on Sept. 19. Not since 2008 have there been so many 20-cent changes. Last year those happened 58 times. In 2011 they happened just 21 times, and in 2010 just 7 times. "There's more and more feast or famine," says Tom Kloza, chief oil analyst at the Oil Price Information Service and GasBuddy.com. The problem, analysts say, is a fuel system increasingly vulnerable to short-term shocks. That's because refiners try to keep stocks of gasoline low to save money, just as other manufacturers aim to operate on a "just-in-time" inventory schedule. The nation has about 26 days' worth of gasoline demand in storage, compared with 30 to 40 days' worth during much of the 1980's and 1990's, according to the Energy Department. Also, there are 143 operating refineries, about half the total from 1980, so, if one has a problem, supplies quickly drop. That price whiplash has a cost. Spikes in gasoline prices are more damaging to the economy than a slow rise in prices because they undermine consumer confidence, economists say. Drivers may be pleasantly surprised when prices slide lower, like they have recently a¿¿ the national average is at $3.28, its lowest level of the year. But they don't know when the price might bounce back up, and increases are almost always sharper than decreases. That makes it harder to budget for the daily commute, or know whether dinner out or a new appliance will be affordable.