In winning his first Nationwide race of the year and the 13th of his career, Keselowski beat runner-up Harvick to the finish line by 1.180 seconds. Busch rolled across the stripe 3.796 seconds behind the winner.
Kasey Kahne came home fourth, followed by polesitter Elliott Sadler, who took the series points lead from 17th-place finisher Reed Sorenson with his fifth-place run. Sadler leads Sorenson by four points 18 races into the season.
Keselowski said calculating how much fuel he was saving is more of a seat-of-the-pants operation, rather than a precise calculation.
"I like to say it's a special gauge we put in the car—I call it an assonometer," Keselowski said. "It's my ass. That's my gauge."
Keselowski said clean air was crucial to the performance of his car.
"I felt like we were pretty good the very first run," Keselowski said. "You never know when you have a race-winning car, especially when it transfers from day into night. I certainly felt more comfortable once I got clean air, and I said something to (crew chief) Todd (Gordon) about it.
"You get that clean air, and these cars are just phenomenal to drive, and mine was no exception. It was really, really fast."
Keselowski, who led a race-high 132 laps, had pulled away to a lead of more than two seconds before Robert Richardson backed into the outside wall on Lap 133 to cause the fifth caution of the race. Taking fuel only, Keselowski was first off pit road, followed by Harvick and three Turner Motorsports Chevrolets, driven by Mark Martin, Justin Allgaier and Sorenson.
With all drivers trying to save fuel after a restart on Lap 139, Keselowski again pulled out to a lead of more than one second and held the top spot the rest of the way, as the race ran green for the final 61 laps.
Ford drivers Carl Edwards, Ricky Stenhouse Jr. and Trevor Bayne all had to make late pit stops for fuel. Stenhouse, third in the series standings and 27 points behind Sadler, left the track frustrated.
"That was a tough night," Stenhouse said. "Our (car) was fast. We had a car to win—we just are getting beat on this fuel mileage stuff every week. We're just going to have to go to the shop and figure it out."