NEW YORK - With the price of gas above $3.50 a gallon in all but one state, there are signs that Americans are cutting back on driving, reversing a steady increase in demand for fuel as the economy improves. For five consecutive weeks, Americans have bought less gas than they did a year earlier, according to MasterCard Spending Pulse, which tracks the volume of gas sold at 140,000 service stations nationwide. For the week of April 1, drivers bought about 2.4 million fewer gallons than they did one year earlier, or 3.6 percent.