Metro has kept a substantial number of the new bus and subway riders it attracted during the gasoline shortage last summer, according to its latest ridership survey released yesterday.

The figures, which report ridership trends through last September, showed that the combined bus and subway system was carrying 26 percent more people than a year earlier. On an average weekday, Metro had 618,014 bus and subway passengers.

New riders, however, have not put enough new money into the farebox to pay the added cost of carrying them, the Metro budget shows. That problem, which faces transit systems across the country, is one of the most curious in public transit economics.

Metro was forced to add 66 new bus trips during last summer's gasoline shortage, which saw ridership increase by as much as 30 percent over the preceding year.

Those 66 new trips required the hiring of more drivers and the consumption of more diesel fuel, for which Metro paid higher prices. The result is that Metro is now forecasting an $11.4 million increase in the local government subsidy required for the current fiscal year -- and most of that comes from added bus service.

Metro budgeted $147.3 million to run its buses this year; it now projects they will cost $163.7 million. Revenues from fares were budgeted at $63.2 million and, because the ridership increase, are now projected at $69 million. That means local governments will have to pay an additional $10.6 million for bus trips.

The subway, on the other hand, can carry more people without having to add employes. Therefore, subway costs, budgeted at $74.6 million, are now projected to be $78.5 million. Revenue was budgeted at $40.9 million and is now projected to be $44 million. Thus the added subsidy is only $800,000 and is a good example for those who argue that subways, despite their enormous construction costs, are potentially more economical to operate than buses.

The ridership increase for September represents one complete transit trip, regardless of whether the rider took a bus or a train or both. Subway ridership alone -- including people who transferred to or from buses -- was up 36 percent in September from the previous year. Some of that increase must be attributed to the Orange Line from Stadium-Armory to New Carrollton, which was not open in September 1978.

In other Metro news yesterday, General Manager Richard S. Page told the board that the subway system has had enough cars for the last two days to run its fully scheduled service. For more than two months, a shortage of parts and other maintenance difficulties have sidelined almost one-third of Metro's cars. Riders suffered more crowded trains that ran less frequently.

The board told Page yesterday that it wanted a report on whether Metro should change the flooring in its new cars from carpeting to some kind of rubberized surface that would be easier to clean. The request was a turnaround for the board, which insisted on carpets when the latest subway car contract was awarded despite vigorous suggestions from the Metro staff that carpet was much more difficult to clean and maintain.