The 22-year-old Allgaier, coming off wins at New Jersey and Talladega, entered the week third in the standings behind Scott Speed and Ricky Stenhouse Jr.
Speed finished 34th Sunday after being parked for intentionally wrecking Stenhouse in retaliation for another wreck. Stenhouse ended up 25th, 69 laps behind.
"This is unbelievable," Allgaier said. "We came in hoping to protect our third- place position in points -- we had Frank (Kimmel) right behind us. We came in with nothing to lose, so we did what we do every week, we raced as hard as we could and went for the win. As things turned out, it also brought our team the championship.
"I'm so proud of all these guys on this crew. They're the reason I'm up here tonight. I'm so relieved this is over. Now we can really focus on the Nationwide races at Texas and Homestead. This really helps with the confidence."
Allgaier, driving his family's No. 16 Hoosier Tire Midwest-AG Tech-Auto Xpress Chevrolet, got underneath Carter 12 laps from the finish and drove away for his eighth career victory in the series.
On lap 27, Stenhouse, running third right behind Speed, got into the back of Speed heading into the third turn. Speed drifted up the track and hit the wall, causing extensive damage to the right side of the car.
"He (Stenhouse) was on my bumper and he pushed me flat out until I hit the wall," Speed said. "And then the car was completely undrivable. It wasn't even close. It was so bad from hitting the wall. I couldn't do anything with it.
"Stenhouse started it and he isn't going to win this championship with that attitude. That was ridiculous. That was the most blatant thing I ever saw in my life."
After losing a lap on pit road, Speed returned to the track. Then, as the leaders, including Stenhouse, drove to the high side of Speed to put him another lap down, Speed turned right as Stenhouse was going by, sending Stenhouse hard into the wall.
"I didn't really expect him to come after me after that, but when I saw him slowing down on the track, I knew that I was in trouble," Stenhouse said.
ARCA officials parked Speed for rough driving for the remainder of the race.
Kimmel finished third in the race, followed by Patrick Sheltra, Tom Hessert III, John Wes Townley, Bryan Silas, Tayler Malsam, Parker Kligerman and Todd Bowsher.
NASCAR Sprint Cup driver David Ragan was 15th, three laps down, in the race slowed by 12 cautions periods that consumed 83 laps.
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