One letter in five mailed in the United States that is supposed to get to its destination overnight arrives late, according to a survey by an independent consultant hired by the U.S. Postal Service.

The consultant, asked to devise a more accurate way of measuring mail delivery from the collection box to its destination, found that 80 percent of the mail that is supposed to arrive overnight actually makes it on time. For years, the Postal Service standard for overnight delivery has been 94 percent, a rate it came close to meeting over the last year under its traditional score-keeping method.

The new measurement system replaced the old methodology in June, and the new scores indicate that mail service is worse than officials previously thought.

In the first survey under the new system, mail within Northern Virginia scored the worst in the Washington area, with only 71 percent of the mail that was supposed to arrive overnight actually making that goal. In the post office's southern Maryland region, which includes Montgomery and Prince George's counties, the rate was 83 percent and in the District it was 81.9 percent.

The scores were for the fourth quarter, June through September.

Local postal officials said they are not satisfied with the scores, do not have an explanation for them and their officers are studying the matter. Postmaster General Anthony M. Frank gave the Postal Service a "low B" for its rating and said, "It's a little bit better than I had thought it would be, but clearly not enough."

Creating and publishing scores from the new system, which measures delivery from collection box to home or business, "is fraught with risk, no question about it," given the expected public criticism that may follow, Frank said. "We have to start somewhere."

For 20 years the Postal Service has used one system -- called ODIS, for Origin-Destination Information System -- to measure whether it delivered the mail on time. Its national standard was that 94 percent of the mail destined for overnight delivery should arrive on time.

In February the Postal Service, believing the old system was neither accurate nor helpful in pinpointing delivery problems, signed a $23.4 million, three-year contract with the Price Waterhouse accounting firm to create and administer a more precise system.

Frank and other postal officials noted that the two systems are different and that the new system measures mail sent to and from households and small businesses that is not imprinted with a bar code by the sender. Only 10 percent of the mail in the country carries bar codes, allowing it to be read and processed by automated equipment.

ODIS measured mail delivery from the time it got into a post office to the time it was given to a letter carrier. The 94 percent delivery standard for overnight mail under that system applied to mail addressed to an area 100 to 150 miles from the sending point. It is supposed to arrive overnight. Fifty percent of all U.S. mail is overnight mail.

Mail sent to locations up to 600 miles away is supposed to take two days. Everything else is given three days to be delivered. Under ODIS, 90 percent of the mail in the two- and three-day delivery areas was supposed to arrive on time. The national scores hovered in the high-80s and low-90s.

The new system, which postal officials did not want to compare with the old system, found that 74 percent of the two-day mail arrived on time and 81 percent of the three-day mail made it on time.

Northern Virginia's two- and three-day scores were 56.5 and 87 percent respectively; southern Maryland's were 63 and 73 percent respectively; the District's were 70 and 77 percent.

"It's a gap we need to improve upon," said Charles Edmiston, head of Montgomery County's mail service.

"The bottom line is, it's such a new system, we really don't know what to make of it," said Robert Faruq, spokesman for the Northern Virginia region, who gave the region's score a "C."

In the past two years, the region has received millions of dollars in new resources to improve its deteriorating service. "We do have a lot more resources now . . . ," said Faruq, "but we needed them two years ago. We were behind the eight ball."