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The Surfers Paradise Street Circuit is not an especially aero-sensitive track at the best of times, but with less than an hour of total practice time today and continuously changing weather during those sessions, it was even more difficult for Mostert to get a read on the new bits.
“It's really hard to really kind of judge it around here compared to the track we just came from, so, between Bathurst and here, completely different,” he said.
“I can't really comment too much on it, to be honest.
“I was surprised at that last lap to be up here – the conditions were pretty tricky – but everyone's kind of dealing with the same thing.
“We had a list of things to try throughout the sessions; we still try did them, but a little bit hard to get a read on.”
Mostert was the only Ford driver in the top four, finishing up 0.1975s off the pace set by Triple Eight Race Engineering's Shane van Gisbergen, but the top six was rounded out by two of the Tickford Racing Mustangs.
Despite not rushing to judgement about the new aero, the two-time Great Race winner did identify some encouraging aspects of his day.
“It looks pretty tight at the top, so that's pretty promising,” he added.
“It's going to be pretty interesting qualifying tomorrow because, personally, I don't feel that confident out there.
“I haven't had a lot of dry laps today and even when you're changing lines, still off-line is a bit wet as well.
“It's still fun, sliding around, trying to commit with the car, and today WAU gave me something that I felt a bit more confident with than I have for the last couple of rounds anyway.”
Qualifying for Race 25 starts tomorrow at 11:35 local time/12:35 AEDT.