In the first hit-out since centre of gravity changes were enforced on the Supercars field, Holden's Commodore has dominated Friday's practice in Tasmania.
Red Bull Holden Racing Team star Jamie Whincup set a new practice lap record of 50.5084 seconds at Symmons Plains to finish ahead of teammate Shane Van Gisbergen (50.5271).
Centre of gravity changes were made on the Commodore and the Ford Mustang following testing after the Melbourne Grand Prix event.
The Mustang has dominated the season so far, winning all six races.
Holden however locked out the top five spaces in the timesheet in Tasmania, with David Reynolds, Mark Winterbottom and James Courtney coming behind the Red Bull pair.
Tickford Racing's Chaz Mostert was the fastest Mustang in sixth on the timesheets with defending champion and standings leader Scott McLaughlin seventh-quickest.
"This is predominantly a Holden track," Reynolds said.
"Holden have dominated here over the years.
"We'll see what happens tomorrow, there's still a lot of sessions to get through."
McLaughlin, who is battling a cold, said his time had more to do with poor driving than any issues with his Shell V-Power Mustang.
"I didn't get the lap together myself," McLaughlin said.
"Always as a driver you want to get out there and do the best.
"Overall I know where to work on, where to get better. It's so close though, so you can't afford to make a little hiccup."
McLaughlin holds a 31-point lead over Whincup but has never won at Symmons Plains.
The action resumes on Saturday with a 50-lap race from 4.40pm AEDT.
Supercars will also trial new parc ferme regulations this weekend, with teams unable to make changes to a car's setup between qualifying and race start.
Mostert said it was unclear what impact the new rules would have.
"What's more important? The race or the qualifying?" the Ford star said.
"We need to make our qualifying car a bit better and then that will make our race car a bit better anyway. Hopefully it doesn't hurt us as much as some other people."
