Sensex jumped 550 points and the Nifty 50 reclaimed the 24,000 mark at midday on Wednesday, snapping a two-day losing streak that had dragged the benchmark nearly 370 points lower. The cavalry arrived from an unlikely corner — monthly auto sales numbers that turned out far better than the market feared.

The Nifty Auto index hit an intraday record high of 26,965.45, surging 1.8%, as every major automaker's June...