In absolutely stunning fashion, Target Chip Ganassi Racing driver Scott Dixon won the GoPro IndyCar Grand Prix of Sonoma, and with it, became a four-time IndyCar Series champion by defeating Juan Pablo Montoya in the final race of the season on a tiebreaker. Dixon, who entered the season finale at Sonoma Raceway third in the championship standings (47 points behind Montoya), was one of six drivers mathematically eligible to win the championship with double points on offer in the final race. Graham Rahal, entering the race second in points, was Honda's lone championship challenger in the midst of a resurgent 2015 season. Defending IndyCar champion Will Power won pole position on Saturday to aid his title bid, along with his Penske teammate Helio Castroneves, and CFH Racing's rising star Josef Newgarden who were both impossible long-shots. The race began with many tributes to the late Justin Wilson, who died Monday evening from injuries sustained in an incident during last week's race at Pocono Raceway. During the pre-race ceremonies, one of several video tributes was shown to recap his stellar career, the British national anthem was played in his honor, and a moments' silence was observed in honor of one of racing's most gracious and beloved drivers. There was even a number 25 drawn into the sky by an airplane. Every car, every driver, and nearly every team member up and down the paddock adorned a decal on their car, or on their helmet, or wore a T-shirt to honor Wilson, and Dixon dedicated his victory at Sonoma to his long-time friend and racing rival. The race took a dramatic turn for Montoya on Lap 39, when he collided with his teammate Power at the entrance of Turn 5. Both would need to pit for repairs under the safety car, leaving Montoya relegated down to 22nd place when the race restarted on lap 42. Meanwhile, Dixon drove to the lead on lap 51 using a combination of expert strategy and well-measured pace, putting him in position to win the IndyCar championship if Montoya finished lower than fifth place. With fifteen laps to go, Montoya had to drive from twelfth to catch and pass fifth-placed Ryan Briscoe. He received help in the form of two collisions in front of him, plus a late drive-through penalty for Sebastien Bourdais. Montoya drove valiantly to chase down Briscoe in the final laps, but he could only close to within 1.2 seconds of Briscoe, and with Dixon winning and leading a race-high 34 laps, Dixon and Montoya finished the season tied at 556 points. However, Dixon's third victory of the season - the thirty-eighth of his illustrious American open wheel career - was the decider, giving Dixon his fourth IndyCar championship. The 35-year-old driver now joins his former teammate Dario Franchitti, Mario Andretti, and Sebastien Bourdais, as four-time champions in the top open-wheel racing series in America. "There was still a chance and that's what I was hoping for," Dixon said after the race. "I don't know what to say. This season we had some big races, and this was the biggest. We were such a longshot." A frustrated Montoya blamed the double-points designation of the race for costing him the championship after he had led from the start of the 2015 IndyCar Season. Ironically, Montoya won the only other double-points race of the season, the Indianapolis 500, and he lost the title in Sonoma in the same fashion that he won the 1999 CART championship - beating Dario Franchitti by winning more races than Franchitti when the two drivers tied in points. He wasn't the only frustrated driver leaving Sonoma. Graham Rahal's comeback season ended with constant handling problems throughout the race and a disastrous 18th place finish after he collided with Bourdais with six laps remaining. Though he ultimately fell to fourth in the standings, Rahal can still take pride in a remarkable 2015 season. Josef Newgarden lost a lap in a disastrous pit stop, taking him out of contention to win the final race of the season. Further down the field, James Jakes thankfully emerged unhurt from a crash on lap 65 when his car spun into a tyre barrier at turn 9. The last race of the season saw the podium completed by Andretti Autosport's Ryan Hunter-Reay, who closed out his 2015 campaign strong with two wins and a second place finish in the finale. Charlie Kimball got his first podium finish since the Indy 500 by finishing 3rd, ahead of Ganassi teammate Tony Kanaan, with Briscoe holding onto the crucial fifth place ahead of championship runner-up Montoya. Power recovered from his tangle with Montoya to finish seventh. Takuma Sato finished eighth, while Dale Coyne Racing's Rodolfo Gonzalez scored his first top-ten finish in ninth, and Mikhail Aleshin returned to the series with a solid tenth-place finish. Veteran driver Oriol Servia drove Justin Wilson's #25 Andretti Autosport Honda to twelfth place in an emotional comeback race for the Spaniard, a long-time friend of Wilson's. Dixon's championship-winning drive further cements the New Zealander's place among the legends of motorsport, and draws a close to a 2015 Verizon IndyCar Series season punctuated by some amazing racing, landmark victories for many of its finest drivers, and overall positive growth for the series as a whole - but marred by tragedy in its penultimate round.