FONTANA, Calif. – After the sun set on Auto Club Speedway, John Hunter Nemechek drove his No. 20 Joe Gibbs Racing Toyota to victory in Sunday’s Production Alliance Group 300 in the final race at the 2-mile track.

It wasn’t supposed to be that way. The NASCAR Xfinity Series drivers were scheduled to race on Saturday, leaving the speedway’s swan song to the NASCAR Cup Series on Sunday, but rain and snow – yes, snow in Southern California’s Inland Empire – forced postponement of the race until Sunday.

RELATED: Official results | At-track photos

And after Kyle Busch won the Cup race, Nemechek, who drove for Busch in the NASCAR Craftsman Truck Series last year, took the checkered flag in his first full-time stint in the JGR Supra.

“Man, this whole 20 team gave me one heck of a race car,” said Nemechek, who took the lead for the last time after a restart on Lap 139 of 150. “Just a huge shout-out to all the guys who work on this 20 team.

“I think a lot of guys doubted (crew chief) Ben (Beshore) coming down from the Cup Series, and to have him on the box and to be able to establish the relationship I have with him, it’s been awesome. I’ve worked with a lot of the guys on the team the last couple of years, and the pit crew – they got it done when it counted tonight.”

Nemechek, who finished second by a matter of inches in the NASCAR Xfinity Series season opener at Daytona, led a race-high 49 laps and finished 0.761 seconds ahead of runner-up Sam Mayer. The Xfinity Series win was the third of his career, and it came at the track where his father, Joe Nemechek, won the first Cup pole of his career and the first-ever pole at the venue.

Justin Allgaier ran third, followed by Chandler Smith. Josh Berry was fifth, joining JR Motorsports teammates Mayer and Allgaier in the top five.

Mayer chased Nemechek for the final 12 laps but couldn’t get close enough to make a move for the win.

“I’m super proud of my team,” Mayer said. “We struggled early on, that’s for sure. We had our work cut out for us, but we threw the kitchen sink at it, and we ended up second. All things considered, I’m really happy and really proud of these guys.”

Cole Custer may have had the fastest car in the race. The driver of the No. 00 Stewart-Haas Racing Ford, the defending race winner, swept the first two stages and led twice for 46 laps. But after a Lap 91 restart, Custer hit the outside wall, thanks to a flat tire, and sustained further damage when the Chevrolet of Austin Dillon couldn’t avoid Custer’s wounded car.

Custer finished 27th, one lap down.

The Xfinity Series’ next race is the Alsco Uniforms 300, scheduled Saturday (4:30 p.m. ET, FS1, PRN, SiriusXM) at Las Vegas Motor Speedway.

Note: Post-race inspection in the NASCAR Xfinity Series garage was completed without issue, affirming Nemechek’s victory in Sunday’s 300-miler.