The final first-round matchups in the "Champions of the 500" bracket challenge are open for voting to determine who will join Helio Castroneves, A.J. Foyt, Johnny Rutherford, Al Unser, Bobby Unser and Bill Vukovich in the quarterfinal round.

Sixteen of the greatest Indianapolis 500 winners – revered names ranging from Meyer to Montoya, Shaw to Wheldon – were selected to square off in a digital voting platform running through May 23, leading to the historic 100th Running of the Indianapolis 500 presented by PennGrade Motor Oil on May 29. The current pairings up for vote pit four-time Indy 500 winner Rick Mears against two-time winner Dan Wheldon and 1969 winner Mario Andretti against three-time champion Dario Franchitti.

Fans can vote for their #Champsofthe500 every hour through noon ET April 30 by visiting ims.com/champsofthe500, and are encouraged to tout their votes by using the hashtag #Champsofthe500 in Facebook and Twitter posts. The voting contest runs in conjunction with Legends Day Honoring Champions of the 500 presented by Firestone at Indianapolis Motor Speedway on May 28.

The most recent bracket challenge voting to close saw four-time winner Al Unser (74.1 percent of the vote) outpoll 1963 race winner Parnelli Jones (25.9 percent) and three-time winner Rutherford (66.9 percent) down two-time winner Al Unser Jr. (33.1 percent). In earlier first-round bracket showdowns, Foyt defeated Juan Pablo Montoya, Vukovich edged Wilbur Shaw, Castroneves topped Louis Meyer and Bobby Unser defeated Mauri Rose.

The Round of 16 features a pair of bracket showdowns running three days each. The eight winners by popular vote advance to the quarterfinal round, with the semifinal and championship rounds to follow. Show your Indianapolis 500 pride and vote for your #Champsofthe500.

#Champsofthe500 First-Round Pairings:

April 18-21: A.J. Foyt def. Juan Pablo Montoya; Bill Vukovich def. Wilbur Shaw

April 21-24: Helio Castroneves def. Louis Meyer; Bobby Unser def. Mauri Rose

April 24-27: Al Unser def. Parnelli Jones; Johnny Rutherford def. Al Unser Jr.

April 27-30: Rick Mears vs. Dan Wheldon; Mario Andretti vs. Dario Franchitti

Note: Voting begins and ends at noon ET on the designated days; drivers receiving the most votes in each pairing advance to the quarterfinal round to follow. Vote at ims.com/champsofthe500.