Chaz Mostert is hoping that forecast rain does not come to pass tomorrow so that he can get the most out of his first Gen3 Supercar test day.

All 25 of the brand-new race cars which are set to line up on the starting grid at next month’s Thrifty Newcastle 500 have now been shaken down, although only nine have formally been put through a test day thus far.

Mostert’s #25 Mobil 1 Optus Ford Mustang is among those to have only completed a shakedown and, while that was a day of unlimited kilometres rather than the usual cap of 60 for a new car, with another private test to come at Winton for Walkinshaw Andretti United before the season starts, tomorrow’s official test day at Sydney Motorsport Park represents precious track time.

However, as at late last night, there is a 95 percent chance of rain today at Eastern Creek and a 90 percent chance tomorrow, when possible falls are two to 15mm, according to forecasts.

Looking at said forecast in more detail, though, the chance of any rain drops to 50 percent as at Wednesday at 11:00 local time, and 30 percent by the 17:00 end of the test day.

Mostert will be hoping most, if not all, of both morning and afternoon are dry, considering the challenge of the Newcastle East Street Circuit awaits in less than three weeks’ time.

Speaking on a hot and sunny season launch day, the two-time Repco Bathurst 1000 winner said, “I’m not sure what the weather’s got in store but [yesterday] was supposed to be raining and we got sun.

“So, if we’re going have a day like this at SMP and a lot of the teams get good running and good testing under their belt, that makes us all feel more confident about going to Newcastle.

“But, when you don’t have a lot of running with this car, you go to one of probably the most I think brutal street tracks we go to – I call it little mini Macau – it’s super tight, the kerbs are aggressive, and with this next generation of car, we have no idea what the durability on a street track like that [will be].

“So, hopefully we get a good run on Wednesday, get another test day in before [the start of the season], and I’m sure we’ll all sleep a little bit easier.”

Supercars gets a new specification of Dunlop wet tyre his year, given the Gen3 vehicles are lighter and generate less downforce than their predecessors, and hence can handle a softer compound.

However, such are testing rules, teams will still be using their tyre banks, including probably what new, dry-weather rubber they are compelled to buy before the first event of the season.

Mostert, meanwhile, is simply looking forward to the task of trying to get on top of not only his own Gen3 Mustang but also his rivals.

“It’s exciting to know there’s a new platform for us to learn,” he added.

“Put 25 guys on the track with several different teams, we all want to beat each other doesn’t matter what we’re driving, so I’m just super-excited [and] they look good.”

Testing starts tomorrow at 08:30 local time/AEDT.