The year a vehicle was produced is, for the most part, not quite the same as the year it was released (which I thought we could go by from now on). The 1997 Tyco Preschool catalog (which I don't have personally, but have seen pictures of on eBay), for example, lists the first six vehicles (which are dated 1996, indicating that they were produced that year) as being new for 1997.
(Have you seen my message regarding Tyco/Fisher-Price catalog scans, by the way?)