Take 2 CEO Strauss Zelnick has stated that the tie-ratio for GTA:IV is the "highest ever". Citing US NPD data through October, the tie ratio was 32% for the PS3 and 28% for the 360 (the latter having a much bigger userbase with a greater proportion of more casual users). That would make the US LTD 1.82m for PS3 & 3.25m for 360 by my reckoning.
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