Jimmie Johnson and Denny Hamilin led the way in Friday's two practice sessions for NASCAR's Sprint Cup-opening Daytona 500.
Six-time champion Johnson will start Sunday's race from the rear of the grid, after having to revert to his spare Chevrolet following a crash in the second Duel qualifying race.
Dual Daytona 500 winner Matt Kenseth, Martin Truex Jr and AJ Allmendinger followed suit, scrapping their primary rides that were already qualified.
Johnson's back-up was the fastest car in Friday's first practice session, turning a 46.372-second lap as one of only 13 drivers to go out.
Hamlin was second in that session, just 0.053s behind Johnson, and then fastest in a busier second session, in which 31 drivers took part.
With a 45.077s lap, Hamlin led Joe Gibbs Racing team-mate and defending NASCAR Sprint Cup champion Kyle Busch, with Truex third.
"The only drafting we did with our primary [car] the other day was with the Gibbs cars, our Toyota team-mates," Truex said.
"We drafted again with the Gibbs cars today in practice and felt our back-up was real similar to the primary.
"It's really hard to tell until you get into racing conditions.
"Hopefully the back-up will act similar in the big pack as our primary did last night in the duel race.
"I was real happy with the primary and felt we had a competitive car."
Truex, who is among the drivers planning to skip Saturday's final practice session, was in the middle of an all-Toyota top five, followed by Carl Edwards and Kenseth.
Kenseth was due to start the 500 on the front row, but will drop from second to the back of the field along with Johnson (who would have started 26th), Truex (28th) and Allmendinger (30th).
"It seemed like the speed was OK," Kenseth said of his back-up car.
"There's not really a big group, and I don't anticipate a big group being out there until they drop the green on Sunday.
"The speed seemed reasonable by itself, I ran with my team-mates a little bit and it seemed OK.
"We were trying to work out a few little things here and there and maybe run a few more times tomorrow and then wait for the race.
"I'd be really surprised if there were very many cars that practiced at all tomorrow, so we'll probably just do single-car stuff and just try to sort out some things we're working on in the car and then get ready to race it."
Ryan Blaney led fellow Ford drivers Joey Logano, Trevor Bayne and Ricky Stenhouse Jr in the second practice session, with Ganassi Chevrolet driver Jamie McMurray completing the top 10.