The US wants to know both spent and unspent convictions. Which is why, when you do the AFP clearance, you pick Code 33 during the process to include all offenses, spent or otherwise. That Australia considers the conviction spent, is irrelevant to US immigration.
As
@SusieQQQ said, the older the conviction, the less likely they'll care - unless it's a crime of moral turpitude. Which speeding isn't. Unless there was an extenuating circumstance involving, for example, an arrest, drink driving, injury or death caused to another. That is, if the speeding was bundled up with a whole lot of other nasty charges.
I wonder if this is a WA thing because I (I'm a Victorian) had one speeding fine back in 1994 which didn't show on the police check at all. I know some other Victorians had speeding fines more recently which also didn't show.
TLDR: I don't think it matters. If you search the forum, there was another Aussie who got asked about her speeding fines and she had a great answer that it was just an infringement and she paid the fine. That seemed to satisfy the CO.
