It's practically conventional wisdom at this point that the Wii has been the best-selling system of this console generation, overall. By sustaining retail sellouts for months after launch and routinely selling millions of consoles every holiday season, the Wii built up what was a seemingly insurmountable lead in the American gaming hardware market, judged purely on raw hardware sales (software sales and hours of play are different matters, of course).

But the conventional wisdom is wrong, at least in the US. Over the past three years, Microsoft has consistently eaten into Nintendo's "insurmountable" US sales lead, to the point that it seems that the Xbox 360 will end the year as the system with the most lifetime sales in America.

In the US, the Wii started out nearly 3 million units behind the Xbox 360 in the sales race, simply by dint of launching a year later. Nintendo's cheaper system quickly ate into that sales margin, though, and it had officially sold more Wiis in the US than Microsoft sold Xbox 360s by June 2008, according to data released by tracking firm NPD and the console makers themselves (the PS3 consistently lagged behind both systems in the US). The Wii sales bonanza didn't slow down from there, and by May of 2010 there were over nine million more Wii units circulating in the country than there were Xbox 360 units.

Unfortunately for Nintendo, this would represent the high-water mark as far as the system's dominance of the American sales charts. For the last three years (starting in June of 2010), the Xbox 360 has outsold the Wii in the US every single month (save for December 2008, when a sales surge led the Wii to beat the Xbox 360 by half a million units). Microsoft's sustained sales dominance in that period has reduced the Wii's American sales lead from over 9 million systems three years ago to just under 2 million units today.

Of course, Microsoft, Nintendo, and Sony will all have new consoles available by the end of the year, meaning time is starting to run out for the current generation of consoles. Will Nintendo be able to essentially run out the clock and maintain its dwindling US sales lead until Microsoft finally stops producing the Xbox 360?

It seems exceedingly unlikely. Over the last 12 months, the Xbox 360 has outsold the Wii in the US by an average of about 278,000 units a month. At that rate, the Wii's current accumulated lead will be completely dried up just in time for Christmas.

Note as well that the Xbox 360 outsold the Wii by 1.75 million units in November and December of last year alone. A repeat of that kind of performance would knock out practically all of Nintendo's current sales lead in one fell swoop. And there's reason to believe the Xbox 360 might have an even more dominant holiday season this time around, since the Xbox 360 still has a robust lineup of games planned for release later in 2013, compared to only a handful of licensed and multiplatform titles on the Wii.

Microsoft also seems poised to pad its US sales lead a little bit more before the Xbox 360 and Wii officially go the way of the dodo. Though Microsoft stopped producing the original Xbox hardware relatively quickly after the Xbox 360 came out in late 2005, Microsoft has said it plans for the Xbox 360 to continue selling for the next five years. That plan might not be so far-fetched; Sony didn't stop producing the PlayStation 2 hardware until earlier this year.

The sales situation in the US is being mirrored in the UK, where tracking firm Chart-Track confirmed today that the Xbox 360 is days away from taking the overall lifetime sales lead in the country, with both systems right around 8.4 million in total sales.

Nintendo fans can take heart, though, that the Wii is maintaining a sizeable sales lead in the rest of the world. That's especially true in Japan, where statistics from tracking firm Media Create (compiled here) show the Xbox 360 has sold a paltry 1.63 million units in the country through June 9, compared to 12.68 million lifetime Japanese Wii sales (and 9.3 million PS3 sales). The Xbox 360 isn't really poised for a US-style sales comeback in Japan either—the system's weekly sales in the country are now measured in the hundreds rather than the hundreds of thousands.

Current, precise numbers for other countries are hard to come by, but official data released by the three console manufacturers in recent months show the Wii as the best-selling system worldwide by a good margin, with somewhere around 100 million worldwide sales (99.84 million in March, to be precise). The Xbox 360 and PS3, on the other hand, are both hovering around three-quarters of that amount (75.9 million for the 360 as of March and 75 million even for the PS3 as of December). As you can see from the above chart, though, those sales are distributed very differently between the various major sales regions for game consoles.

Will either Sony or Microsoft be able to take the worldwide sales crown from Nintendo in the next few years? It seems unlikely. Then again, it seemed unlikely back in 2010 that the Xbox 360 would ever be able to eliminate the Wii's sizeable sales lead in the US. Now, that seems inevitable.

Do these horse race numbers really matter to the health of the three major console makers going forward? Not really, at a granular level. Still, it's interesting to see just how the trajectory of the concluding console generation has changed substantially over time and how much the competitive environment varies in different parts of the world. Plus, it's always fun to claim bragging rights for your console of choice.