Preparations for the new season, and the introduction of the Gen3 rules, ramped up at Sydney Motorsport Park midweek with all 25 cars running together for the first time.
For Walkinshaw Andretti United and drivers Mostert and Nick Percat it was the first test day proper, the team having only done a single shakedown with its Gen3 Ford Mustangs at Winton last week.
The pair ended the day in the second half of the timesheet, Mostert the best of the two, and second-best of the Ford Mustangs, in 13th amid accusations of sandbagging by Blue Oval teams.
Reflecting on the weather-affected day, Mostert admitted there's work to do when it comes to unlocking speed from the Gen3 package, which features a dramatic drop in downforce compared to the Gen2 cars.
"I'm still not exactly sure how to extract the speed from the car, but I felt a bit more comfortable than I did the first day," he said.
"It's like anything, the [ergonomics are] so different inside the car because it's a different chassis, so we've still got to make some changes there.
"We played around with a little bit of set-up. We're still hunting, for sure. Who knows where it will end up.
"Hopefully we get another [test] day before Newcastle. At least running [at SMP] gives us some more ideas so we can try and change our philosophies and keep hunting."
When pressed on whether his comfort issues were ergonomics, a consistent issue for taller drivers throughout the Gen3 development, or car behaviour, Mostert added: "First comfortable, and then there's the comfort of trying to find the limit of the car.
"With losing the aero, its made the car a bit more numb. So the set-up has to change. You can't run last year's set-up and expect it to work.
"These things are going to take time and all-in-all, it was a successful test. In the middle of the day I didn't think it would be, but then I got to try a couple little things that made it worse in some area, and made it better in others.
"It gives us some direction."
Percat was just 24th fastest at the test after his final run was scuppered by a loose wheel.
Both cars are expected to take part in a final test day at Winton next Wednesday.
