Tesla Inc. was the biggest drag on the S&P 500 Index in its first day of trading on the benchmark. The electric-vehicle maker, which now represents 1.6% of the index and ended the day as its sixth heaviest-weighted stock, fell as much as 7% as it retraced gains from Friday when tens of millions of shares were purchased by index-fund managers. Institutional buying of Tesla surged late Friday as index-tracking managers rushed to add the shares to their funds ahead of its index inclusion. Almost $60 billion worth of stock changed hands at $695 a share.