President Carter is changing the timing of his defense budget requests, apparently to make the increase between this year and next look bigger than it really is.

Requests for extra billions to cover inflation, rising fuel costs and Indian Ocean operations will be lumped mostly into the fiscal 1981 budget instead of this year's.

This promies to widen the gap between the two budget years, making it easier for the president to make good on his pledges to provide "real growth" in the Pentagon budget.

However, to give himself more room, the president last night in his economic address limited himself to restating the pledge to NATO allies to increase defense spending by 3 percent a year after allowing for inflation. He did not embrace the 5.4 percent "real" increase in budget authority between fiscal 1980 and 1981 that the Pentagon touted in January as the administration tried to win senators over to the SALT II treaty.

Said Carter last night:

"I will maintain my commitment to a strong defense and to the level of real growth in defense spending which we pledged to our NATO allies. But the Defense Department will not be immune from budget austerity.

"In particular," Carter continued, "I will require that the department make savings that do not affect our military readiness. I consider the proposed defense budget adequate to meet out nation's needs. We must maintain budget restraint and fiscal responsibility in all government agencies."

Carter's propsed military budget for fiscal 1981 calls for $159 billion in new money -- budget authority -- and $143 billion in actual spending.

Air Force Gen. David C. Jones, chairman of the Joint Chiefs of Staff, went on record before the House Budget Committee as saying this would not be enough -- that the services wanted more.

Several senators and representatives of both parties have vowed to add enough money to the new Pentagon budget to provide the 5.4 percent real increase advocated in January. They will argue their case with the help of military leaders summoned to hearings to state their needs, assuring a new round in the battle of the Pentagon budget.

Defense Secretary Harold Brown told some key lawmakers over the last several days that the pentagon will have to absorb some of the extra costs of fuel as well as the other requests for additional money from the military services.

Fuel costs not covered in existing Pentagon budgets are expected to total about $3.6 billion in fiscal 1980 and $4.5 billion in fiscal 1981. Additional money also will be needed to finance military pay raises.

All this is bad news to military leaders who thought this was the year their money requests would have easy sledding inside the Pentagon, White House and Congress. There are growing signs that the "anything for defense" fervor generated by Iran and Afghanistan is receding and worries about inflation advancing.

The Air Force had hoped to get an extra $3.5 billion in fiscal 1980, the Army $850 million, and the Navy $300 million. Civilian leaders have directed the services to make offsetting reductions for most of those requests, as part of the White House's austerity drive.

For fiscal 1981, the military wish l ists for extra money on top of the $159 billion Carter has requested go sky high. in one recent briefing given congressional staffers, the Air Force alone said it would need an extra $10 billion.

Carter's message last night reduces requests of that magnitude, for the moment at least, to wishful thinking as far as the administration, if not the Congress, is concerned.