Who, what and why to watch at the 91st Indy 500

	After qualifying for the pole this year, Helio Castroneves has a clear view of capturing his third Indianapolis 500 trophy this Sunday. Gavin Lawrence/Getty Images

This Sunday marks the 91st running of the Indianapolis 500. To get you caught up on what, and who, to watch, SI.com's Tim Tuttle offers a brief primer to "The Greatest Spectacle in Racing."

Five drivers to watch

1. Dan Wheldon: He's a co-favorite. Wheldon believes he should be trying to win his third straight at Indy. He drove from 16th to victory in 2005. He led 148 laps last year and was making a move to the front when a cut tire forced him to pit. He lost track position en route to finishing fourth. He also was third in '04. Indianapolis is far different than the three ovals the IndyCar Series has raced on this year, but it's still notable that Wheldon has two wins and a second and has led 482 out of 600 laps.

2. Helio Castroneves: He's the other co-favorite. Castroneves won back-to-back at Indy in '01 and '02 and was a very close second in '03. The charismatic Brazilian was knocked out last year in a crash, his first in the race, and underpowered cars kept him out of the hunt the previous two years. At the age of 32 and in his seventh 500, Castroneves is in his prime. Plus, he's starting from the pole and drives for Team Penske, winner of 14 Indy 500s.

3. Sam Hornish Jr.: Team owner Roger Penske compared him to four-time Indy winner Rick Mears after last year's race, in which Hornish drove in Mears' style for the first time and won the race. Hornish never showed all his speed until the closing five laps. He needed it to overtake Scott Dixon, Michael and Marco Andretti after a botched pit stop put Hornish back. Hornish hasn't shown the blazing speed in practice this year that he did in '06, but he'll use the other part of Mears' formula -- improving the car during pit stops -- to be a contender.

4. Danica Patrick: She's driven impressively in her two previous Indy 500s, finishing fourth in '05 and eighth last year in the difficult Panoz chassis, and has been a consistent top-five in practice this May. Patrick knows how to go fast at Indianapolis, her best track. It's a knack most drivers don't have. She has the speed, experience and team to win the race.

5. Tony Kanaan: He's been a front runner in all five of his previous Indy 500s, leading in each race, including 12 laps last year. Kanaan finished fifth, second, eighth and fifth in the last four at the Brickyard. This year Kanaan, who's never started farther back than the second row, starts in the middle of the front. Always fast, he should be in position to make a run for the win at the end. The question is: Will he finally do it?