comparable prior year period. The increase in gross margin was impacted by a 42 basis point ($17.0 million) non-cash LIFO benefit, with the remaining increase primarily from higher merchandise margins.
Operating, selling, general and administrative expenses for the twelve weeks ended May 6, 2023 were $1.3 billion compared with $1.2 billion during the comparable prior year period. As a percentage of sales, these expenses were 31.5% compared with 31.6% during the comparable prior year period.
Net interest expense for the twelve weeks ended May 6, 2023, was $74.3 million compared with $41.9 million during the comparable prior year period. Average borrowings for the twelve weeks ended May 6, 2023 were $7.2 billion, compared with $6.0 billion for the comparable prior year period. Weighted average borrowing rates were 3.85% and 2.74% for the twelve weeks ended May 6, 2023 and May 7, 2022, respectively.
Our effective income tax rate was 17.4% of pretax income for the twelve weeks ended May 6, 2023, and 20.3% for the comparable prior year period. The decrease in the tax rate was primarily attributable to an increased benefit from stock options exercised during the twelve weeks ended May 6, 2023. The benefit of stock options exercised for the twelve week period ended May 6, 2023 was $46.7 million compared to $21.1 million in the comparable prior year period.
Net income for the twelve weeks ended May 6, 2023 increased by $55.2 million from the comparable prior year period to $647.7 million due to the factors set forth above, and diluted earnings per share increased by 17.5% to $34.12 from $29.03. Excluding the non-cash LIFO benefit, adjusted net income increased 7.1% to $634.7 million, and adjusted diluted earnings per share increased 15.2% to $33.43. The impact on current quarter diluted earnings per share from stock repurchases since the end of the comparable prior year period was an increase of $0.78 per share.
Thirty-Six Weeks Ended May 6, 2023
Compared with Thirty-Six Weeks Ended May 7, 2022
Net sales for the thirty-six weeks ended May 6, 2023 increased $862.7 million to $11.8 billion, or 7.9% over net sales of $10.9 billion for the comparable prior year period. Total auto parts sales increased by 7.9%, primarily driven by an increase in domestic same store sales of 4.2% and net sales of $207.1 million from new stores. Domestic commercial sales increased $311.3 million to $3.1 billion, or 11.2%, over the comparable prior year period.
Gross profit for the thirty-six weeks ended May 6, 2023 was $6.1 billion, compared with $5.7 billion during the comparable prior year period. Gross profit, as a percentage of sales, was 51.6% compared to 52.4% during the comparable prior year period. The decrease in gross margin was driven by a 63 basis point ($74.0 million) non-cash LIFO net charge driven primarily by freight costs.
Operating, selling, general and administrative expenses for the thirty-six weeks ended May 6, 2023, were $3.8 billion compared with $3.5 billion during the comparable prior year period. As a percentage of sales, these expenses were 32.5% compared with 32.6% during the comparable prior year period.
Net interest expense for the thirty-six weeks ended May 6, 2023, was $197.6 million compared with $127.6 million during the comparable prior year period. Average borrowings for the thirty-six weeks ended May 6, 2023 were $6.8 billion, compared with $5.6 billion for the comparable prior year period. Weighted average borrowing rates were 3.67% and 3.03% for the thirty-six week periods ended May 6, 2023 and May 7, 2022, respectively.
Our effective income tax rate was 19.0% of pretax income for the thirty-six weeks ended May 6, 2023, and 20.6% for the comparable prior year period. The decrease in the tax rate was primarily attributable to an increased benefit from stock options exercised during the thirty-six weeks ended May 6, 2023. The benefit of stock options exercised for the thirty-six week period ended May 6, 2023 was $89.8 million compared to $55.9 million in the comparable prior year period.
Net income for the thirty-six weeks ended May 6, 2023 increased by $44.0 million from the comparable prior year period to $1.7 billion due to the factors set forth above, and diluted earnings per share increased by 12.0% to $86.10 from $76.90. Excluding the non-cash LIFO net charge, adjusted net income increased 6.2% to $1.7 billion, and adjusted