believe net sales have been positively impacted by other factors, including macroeconomic factors and the number of seven year old or older vehicles on the road. The average age of the U.S. light vehicle fleet continues to trend in our industry’s favor. According to the latest data provided by the Auto Care Association in the 2021 Auto Care Factbook, for the ninth consecutive year, the average age of vehicles on the road has exceeded 11 years. Since the beginning of the fiscal year and through March 2021 (latest publicly available information), miles driven in the U.S. decreased 7.2% compared to the same period in the prior year; however, the March 2021 data showed significant improvement in the number of miles driven. We believe the increase in miles driven is due to the nation beginning to return to pre-pandemic levels, but we are unable to predict if the increase will continue or the extent of the impact it will have on our business.
Twelve Weeks Ended May 8, 2021
Compared with Twelve Weeks Ended May 9, 2020
Net sales for the twelve weeks ended May 8, 2021 increased $871.7 million to $3.651 billion, or 31.4% over net sales of $2.779 billion for the comparable prior year period. Total auto parts sales increased by 31.8%, primarily driven by an increase in domestic same store sales of 28.9% and net sales of $51.5 million from new stores. Domestic commercial sales increased $254.8 million to $828.6 million, or 44.4%, over the comparable prior year period.
Gross profit for the twelve weeks ended May 8, 2021 was $1.915 billion, compared with $1.491 billion during the comparable prior year period. Gross profit, as a percentage of sales was 52.4% compared to 53.6% during the comparable prior year period. The decrease in gross margin was primarily driven by the accelerated growth in our Commercial business and our investment in pricing initiatives.
Operating, selling, general and administrative expenses for the twelve weeks ended May 8, 2021 were $1.111 billion, or 30.4% of net sales, compared with $999.0 million, or 35.9% of net sales during the comparable prior year period. The decrease in operating expenses, as a percentage of sales, was driven by strong sales growth and approximately $75 million in pandemic related expenses, including ETO for our AutoZoners, incurred in the prior year.
Net interest expense for the twelve weeks ended May 8, 2021 was $45.0 million compared with $47.5 million during the comparable prior year period. The decrease was primarily due to a decrease in the weighted average borrowing rate. Average borrowings for the twelve weeks ended May 8, 2021 were $5.351 billion, compared with $5.460 billion for the comparable prior year period. Weighted average borrowing rates were 3.3% and 3.4% for the quarter ended May 8, 2021 and May 9, 2020, respectively.
Our effective income tax rate was 21.4% of pretax income for the twelve weeks ended May 8, 2021, and 22.8% 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 8, 2021. The benefit of stock options exercised for the twelve weeks ended May 8, 2021 was $16.0 million compared to $1.1 million in the comparable prior year period.
Net income for the twelve week period ended May 8, 2021 increased by $253.3 million to $596.2 million from $342.9 million in the comparable prior year period, and diluted earnings per share increased by 84.0% to $26.48 from $14.39. 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 $1.77.
Thirty-Six Weeks Ended May 8, 2021
Compared with Thirty-Six Weeks Ended May 9, 2020
Net sales for the thirty-six weeks ended May 8, 2021 increased $1.630 million to $9.716 billion, or 20.2%, over net sales of $8.086 billion for the comparable prior year period. Total auto parts sales increased by 20.4%, primarily driven by an increase in domestic same store sales of 19.0% and net sales of $133.4 million from new stores. Domestic commercial sales increased by $410.6 million, or 23.4%, to $2.163 billion.
Gross profit for the thirty-six weeks ended May 8, 2021 was $5.150 billion, or 53.0% of net sales, compared with $4.358 billion, or 53.9% of net sales, during the comparable prior year period. The decrease in gross margin was primarily driven by the accelerated growth in our Commercial business and our investment in pricing initiatives.