Earnings of Sears Roebuck & Co., the country's biggest retailer, fell 11.2 percent last year to $2.54 a share from $2.86 on 3.4 million more shares in fiscal 1979, the company reported yesterday.

Net income for Sears, whose fiscal year ended Jan. 31, tumbled to $810.008 million on sales of $17.51 billion for $921.52 million in the previous year on sales of $17.94 billion.

Fourth-quarter sales were up 3.5 percent to $5.10 billion from $4.93 billion a year ago. It was the first time fourth-quarter sales have topped $5 billion; however, net income for the quarter fell to $254.33 million (80 cents a share), off 21.6 percent from the $329.88 million, ($1.02) earned a year earlier.

Chairman Edward R. Telling said high interest costs as well as lower sales caused the profit shrinkages.

He estimated that high interest costs reduced earnings for the year by 16 cents a share and those for the quarter by 6 cents a share. Other expense escalations caused by inflation and a bulge in pension costs also had adverse impacts on earnings, Telling said.

The Allstate Insurance Group contributed $1.45 a share to fiscal 1980 earings, up from $1.44 a share the previous year.

A 7.1 percent gain in sales to $3.764 billion increased net income for Jewel Cos. Inc. to $50.69 million ($4.54 a share) in the fiscal year ended Feb. 2 from $41.14 million ($3.95) a year earlier.

Fourth-quarter sales of $956.52 million were only marginally higher than those of a year earlier, and net income was $19.53 million, ($1.75) compared with $16.64 million ($1.47).