Sears, Roebuck & Co.'s net income for the three months ended July 31 rose 51 per cent to a record $207.1 million (65 cents a share), the nation's largest retail said yesterday.

The figures compared with $137 million (43 cents) earned in the second quarter last year, the company said.

Sears said net income for the first half was $361.2 million ($1.13), another record up 63 per cent from earnings of $222.1 million (70 cents) in the first half of 1976.

A company spokesman attributed the rise in earnings to increased sales and a tightening of expenses as a result of last year's corporate reorganization. Income from the Allstate Insurance division also rose.

Net sales for the second quarter increased 16.3 per cent t0 $4.1 billion from the previous record of $3.5 billion set a year ago, while first-half sales climbed 14.6 per cent to $7.7 million from $6.7 billion in the first half of 1976.

Despite record sales, Firestone Tire & Rubber Co. earnings rose only lightly in the fiscal third quarter due to pressure from domestic price competition and foreign currency exchange losses.

Firestone said net income in the quarter ended July 31 was $22.3 million (39 cents a share) on sales of $1.1 million, compared with $21.5 million (38 cents) on sales of $994.1 million a year earlier.

For the first three quarters, net income for the nation's second largest tire market rose to $84.9 million ($1.48) on record sales of $3.2 billion from $79.9 million ($1.40) on sales of 2.9 billion a yer earlier.

Officials said a $5 million currency exchange loss in the third quarter was at times that of the previous year, adding that one major factor was duly's relative weakness of the U.S. dollar. And over the nine months, exchange losses totaled $2 million compared with a $9 million gain in the same period last year, Firestone said.

European sales were slow, with operating losses in some subsidiaries, but there was some improvement toward the end of the quarter, a spokesman said. Officials said sales and operations in South America remained strong.

Domestic operations generally contributed significant earnings, but Firestone's Dayton and Seiberling division performances were below par "because of severe price competition," executives said.

Deere & Co., a farm industrial machinery manufacturer, earned $1.28 a share in its third quarter, up from $1.26 a year ago, as sales spurted to $1.008 billion from $887 million. Net income was $77.14 million against $75.87 million.

Profit in the nine months ended July 31 was $209.33 million ($3.48 a share) on sales of $2.626 billion compared with $205.5 million ($3.44) a year earlier on sales of $2.149 billion.

Chairman William A. Hewitt said the return on sales dropped to 8 per cent from 8.5 per cent a year ago mainly because of less favorable sales mixes and losses on foreign exchange transactions.

But results in the United States and Canada were more favorable than last year because industrial machinery sales more than offset lower profit margins on farm machinery.