The only two Zirconium vehicles seen in the game are the Stratum, which resembles a 1994-1997 Honda Accord station wagon, and the Journey, which resembles a Vogue '26 Villa Grand. The Zirconium name may be a reference to the Ford marque Mercury, as both share the names of elements, however the only Mercury analogues seen in the games is the Solair, produced by Willard, and the Stretch and Admiral, produced by Dundreary. It is possible that this is a reference to Diamond-Star Motors, a joint venture between the Chrysler Corporation and Mitsubishi Motors Corporation.
In GTA IV 's audio files, this manufacturer is misspelled as "Zireonium" and is mispronounced by the police as "Zireoenium". Interestingly enough, a sound clip with the same mispronounciation can be found in GTA V's files that was probably recorded before being corrected.
Zirconium is a frequently used material for making cheap diamonds. This is maybe a reference to the Stratum's low impact resistance and cheap quality.
It could be based on either Saturn or Subaru since the Stratum carries notable influences from both the 2nd gen Saturn SW1 and 1st gen Subaru Outback. It, however, resembles as the Honda Accord overall, thus bearing little or no connection to either of the real-life brands.
It could be based on the defunct Ford-owned Merkur brand since it only offered 2 models such as the Merkur XR4Ti and the Merkur Scorpio.