The nation's largest gasoline marketer, Shell Oil Co., said yesterday it will begin rationing gasoline supplies to its wholesale and retail dealers beginning today, limiting them to 75 percent of their usual gasoline deliveries.

Shell's announcement, which was followed by a statement from Texaco Inc. that some of its gasoline supplies were "extremely tight," does not mean that there will be a national gasoline shortage and long lines at gas stations as there were during the 1973-1974 Arab oil embargo.

Shell spokesman Tom M. Denman said the Houston-based company has begun to ration gasoline to its sales outlets "because over the last few weeks we have had a problem supplying them with unleaded gasoline."

Unleaded, high-octane gasoline is required by most of Detroit's recent model cars to meet federal air emission and mileage efficiency standards required by law.

Asked about the possibility of a national gasoline shortage, Jack Blum, a representative of the Independent Gasoline Marketers Council, said, "The overall market is okay; the shortages are really selected market dislocations limited to some companies and mostly for unleaded gas."

Another factor has been a spell of good weather, which led to an unusual amount of driving. Shell said yesterday that an unseasonal "dramatic increase in demand" was a consideration in its rationing plan. Shell also cited federal "regulations which have held down" its prices to an "unrealistically low level."

The oil industry and the Department of Energy have been pressing hard to remove price controls from gasoline that have been in effect since 1973. President Carter is expected to send a price decontrol request to Congress in January, which, if enacted, would raise gasoline prices by about 4 cents a gallon.

Under current regulation, oil companies cannot pass through the costs of upgrading refineries to produce high-octane unleaded gasoline that the Environmental Protection Agency requires.

In its weekly report on fuel, the American Petroleum Institute said that the country's fuel stocks totaled 217.6 million barrels as of Nov. 24, up 5 million barrels above the previous week but down about 32.5 million barrels from last year. There are 42 gallons in a barrel.

DOE has given Shell authority to allocate or, in effect, ration its gasoline until Dec. 12, and the Energy Department is considering extending the rationing authority through the end of the year.

In an 18-page request for approval for its allocation plan filed with DOE's Office of Hearings and Appeals, Shell said the anticipated shortages of its unleaded gasoline were the result of DOE pricing regulations that forced the oil company to undersell its competitors. Industry-wide, demand for gasoline was up 2 to 3 percent in November compared with last November. Demand for Shell gasoline has increased by 14 percent.

Shell also said that it was forced to lower its production of unleaded gasoline due to unexpected maintenance at two key refineries in Wood River, Ill., and at Norco, La.

Despite the lower allocations of gasoline to dealers starting today, Shell says it will also be lowering gasoline prices by a half-cent a gallon in accordance with DOE regulations.

A Shell Oil executive said that despite plans to allocate gasoline to outlets, the company hopes to be back to normal supply patterns by early January. Last year the company, which has 8 percent of the U.S. gasoline market, sold 740 million gallons of gasoline. Even with allocations, Shell says it still expects to sell about the same amount.

Shell's DOE-approved allocation plan would limit supply outlets to 75 percent of the gasoline they sold last December or, if the total is greater, 75 percent of 1972 December sales.

Yesterday, in a related action, four New Jersey Shell dealers filed a suit in Newark's U.S. District Court alleging that Shell "discriminates" against dealers who offer mechanical repairs in favor of "gas-only" stations alleged to receive discounts.