"THIS WEEK, no less an authority than Federal Reserve Board Chairman Alan Greenspan suggested that many Americans may have the wrong mortgage.</br></br>In a speech to credit unions, Mr. Greenspan said that Federal Reserve Board research showed that ""many homeowners might have saved tens of thousands of dollars had they held adjustable-rate mortgages rather than fixed-rate mortgages during the past decade"" as rates tumbled.</br></br>Yet the more expensive fixed-rate loans accounted for 81% of mortgage originations last year, according to the Mortgage Bankers Association. That's because most borrowers are risk-averse and fixed- rate mortgages protect them from the possibility that interest rates will rise over the life of the loan.</br></br>The problem is that as home prices soar, choosing a long-term loan is an increasingly costly decision. In the current market, borrowers can cut the rate on their mortgage drastically by opting for an adjustable-rate mortgage instead of a 30-year fixed-rate loan. Currently, rates average 5.71% on 30-year fixed-rate mortgages while they average just 4.53% on mortgages that carry the same rate for the first five years and then adjust yearly, according to HSH Associates, a financial publisher.</br></br>Lenders are increasingly pushing ARMs because they allow buyers to either trim their payments or buy more home for the same payment. At Washington Mutual Inc., ARMs accounted for 55% of home loan applications in the fourth quarter. At a time of rising home prices, the lower start rates on ARMs allow borrowers ""to maximize their buying power,"" says Greg Sayegh, Washington Mutual senior vice president."