The Heat's Game 7 win over the San Antonio Spurs drew the second-highest preliminary television rating for the NBA Finals on ABC since the network took over the series in 2003.
Miami's 95-88 victory Thursday night earned a 17.7 overnight rating. Game 7 between the Los Angeles Lakers and Boston Celtics in 2010 had an 18.2 overnight.
The series averaged a 12.4 overnight, up 5 percent from last year's Heat-Thunder finals, which went only five games.
Ratings represent the percentage of all homes with televisions tuned to a program.
Overnight ratings measure the country's largest markets.