Buoyed by the increasing tendency of Americans to dine out, the nation's restauranters predict sales in their industry will top $100 billion next year, up 10.2 percent over this year.

Leading the sales surge are are fastfood restaurants. Such eating places, which now comprise the secon largest sector in the industry behind restaurants and lunchrooms, alone are expected to account for sales of $22 billion in 1979, according to the National Restaurant Association.

"One-third of the people in our country eat out at least once a day," Thad A. Eure, president of the association, said in a recent speech. "More restaurants than gas stations are providing Americans with a variety of ways to eat away from home wherever they are and at whatever price they desire to pay.

The industry's optimistic forecast contrasts with the generally pessimistic outlook for the economy as a whole. Many economists and business people expect the economy and personal incomes to grow at considerably lower rates next year than this year.

But to offset this, restaurant owners are counting on an increase in the number of working wives (approximately 49 percent currently) to prompt families to eat out more often. Also, industry members contend that a general economic slowdown, while likely to depress purchases of houses and other big-ticket items, tends less to affect smaller purchases such as restaurant meal.

Still, the restaurant association's most recent newsletter warns that the industry may be headed for tougher times. "As economic expansion slows in 1979, the sales performance of eating places will depend to a greater extent on the merchandising and marketing skills of restaurant operators," the report says.

The strength of television advertising is expected to be a major factor in the continued growth of the food service industry . . . Other marchandising and marketing skills which are increasingly applied included the effective use of promotions, the restructuring of menus to offer meals in all price ranges and the upgrading of restaurant decor."

A large part of the dollar sales increase predicted for next year is attributed to inflation.

Eure noted that these projections were prior to President Carter's announcement of more stringent wage and price guidelines. Eure said he could not predict what effects the new guidelines might have on the new forecasts.