I don't see a problem with my Gears predition really.

Gears 2 opened up to 1.9 Million units in 2008, when there were 21 Million 360's on the market. We've had nothing Gears since then on any platform. So I'm guessing the increase in userbase as well as whoever tried the game out 2nd hand will increase potential userbase for Gears 3.

That would be true if it wasn't for the fact that the majority of sequels suffer either lower or very similar FW sales to their predeccesors...

Those sales are nothing to be sniffed at, but with no confirmed release date (Could launch in summer drought, on same day as Skyrim and COD, same day as SS etc.), it's unfair to make an accurate prediction. But pre-orders, well, they're certainly impressive for a game with no release date!

So you expect Skyword Sword to be the highest selling Zelda ever other than Ocarina of Time? Twilight princess only did 5.8 million I don't really see much reason why Skyword Sword will do more than 6.5 million maybe 7 at most lifetime.  Can you please explain to me where the huge boost in the series popularity will com from?

Gears 3 will outsell the rest of the franchise for pretty obvious and simple reasons.

1.  Every "large scale" shooter this gen has increased its sales with each iteration due to the spread of online multiplayer and word of mouth about playing competitive online.

2.  It features more modes, characters, and just about everything than any previous Gears title.

3.  Many who purchased the game pre-owned are now fans and will increase the games initial buys.  Don't be fooled, every single game in the database has been sold to FAR more people than VgChartz can track by due to used sales.

4.  Its MS flagship title for the holidays and will get an IMMENSE marketing campaign probably along the lines of what Halo 3 did.  MS marketing has pushed sales of just about everything they've poured serious advertising into and Gears will be no different.

5.  Cliff and Epic have already announced this is the end of the story of Delta Squad and that while there may be more games inthe Gears universe this will be the end point of the trilogy and their story.  Hardcore fans will be buying the heck out of it to see how it all ends for Marcus and Dom.

No, I expect SS to sell around the same as TP, i.e. the Wii and GC versions combined

So I reckon around 7.5mil at the end, due to a higher install base and the graphical style appeals to the Wii's audience

(Just because it's the last in the trilogy doesn't mean it'll neccessarily sell better, Halo:Reach hasn't...)

And lots of games have had more features, better ad campaigns, higher install base, and have either done worse or the same. Galaxy 2 is a good example, it'll end up around the same as the original (10mil). That's awesome sales, but with everything going for it, why weren't there more?

Couple problems with your post.

1st.  Halo Reach has sold more than Halo 3 did in the same amount of weeks.

2nd.  Obviously there was overlap between the Gamecube and Wii versions of Twilight Princess. 

3rd.  Galaxy 2 isn't comparable to Gears at all in terms of increasing userbase.  Galaxy is a sinlge player driven experience only and isn't going to significantly outsell its predessor due to the fact that in a way its more of the same.  Granted that is an EXCELLENT platforming experience but still should sell in line with Galaxy 1 due to there not being an extensive change in gameplay or forumla.  Gears 3 will outsell Gears 2 due to the expansion of Online multiplayer.  Every sinlge other large scale shooter this gen has outsold its predessesor and its for one main reason only, the online capabilities.

Of course, this is all just speculation ATM

Can you give me some examples? I can give you many examples of the opposite happening... Here's a few.

Gears 2 > Gears 1
Uncharted 2 > Uncharted 1
Black Ops > Modern Warfare 2 > Modern Warfare
Dead Space 2 > Dead Space

There are many more, but I decided to use more blockbuster titles, as that's what these games are. I would also like to say that Epic/Microsoft aren't stupid enough to release Gears of War the same Week as Call of Duty. And it's much bigger than Elder Scrolls, and it's releasing during "Fall", not Summer.

Gears week 1 easily.

Then it's between Zelda and Gears for lifetime. Gears will probably take a massive early lead though.

Elder Scrolls is 11/11/11, I expect the same thing for COD. And it's easy to push a game with no release date back...

All those games, bar Black Ops. were new entries, and bar UC, were very close in sales. Dead Space is too soon...

I suppose LBP2 and LBP is one, those are pretty damn close ATM

GT5 is another, falling behind GT4...

Halo:Reach is only just ahead, with a much bigger usebase...

It could go either way, depends on launch, of course

Does it matter if they're new entries or not? I don't think so... They're still sequels of high quality games lol.

And does it also really matter if they're close? No. lol. They are still selling more

LBP2 has sold 780,000 units in 3 Weeks, in January... Without Japan data. LBP sold 730k in 3 Weeks, in November... With Japan data. It's tracking above LBP so I don't understand what you're getting at with that.

GT5 is currently above GT4 by quite a bit, despite seling less this Week in comparison to GT4's Week.

Reach is still ahead of 3.

I'm still waiting for you to show me a big blockbuster game where the sequel has done less