MasterCard's report shows drivers bought 2.7 billion gallons of gas last week, down 3.6 percent from the same period in 2010, when it was 80 cents cheaper.

The decline is somewhat puzzling because Americans typically curb their driving only as a last resort, after sacrificing other forms of discretionary spending, like shopping for new clothes, or going to movies, concerts and restaurants.

But demand for gas is falling while other types of spending are on the rise. Retail sales rose 2 percent in March compared with a year earlier, surprising economists who were expecting no increase or even a decline.

Gamel said it's too early to tell whether this is the kind of long-term decline in demand that the economy endured during the recession. Prices already are in the range when Americans started to leave their cars in the driveway several years ago. Drivers began to cut back on gas in October 2007, when the national average approached $3 per gallon.

Even if demand for gas keeps falling in the U.S., it probably won't be enough to force the price down. That's because worldwide demand for crude oil keeps rising.