Yes.. of course I know out the top of my head how the old GTs sold... >_<"

How are you meant to have proof of a sales prediction?

I think most of us already know the answer to that.

Honestly, GT5 is just about guaranteed to be the 2nd best selling GT title as far as I can tell. I can easily see developing markets such as those in South America and the Middle East giving about a million additional sales to GT5 over GT4. I expect Europe will be higher thanks to the introduction to popular European brands such as Ferrari, Lamborghini, Maserati, and McLaren as well as WRC and Top Gear. Japan I expect to stay about the same. America I expect to make the biggest jump in sales over GT4 actually largely thanks to NASCAR and the new damage engine. As far as becoming the best selling GT game, that will all depend on how heavily Sony bundles it. Honestly, the best thing that GT5 has going for it is the realistic chance that it might be the only main Gran Turismo game this generation. With no sequel to cut the legs off sales, it might continue selling well longer than past games.

Well if you read what he says, he speaks in certainty. 

It's implied that it's his opinion.

I see no implication to make me think that.
And if you read it, it quite clearly is written in certainty.

"But all the Delays hurt it."

This.

GT5 will be the best selling Gran Turismo after end of PS3 gen.

 Look at it this way, now GT5 has less time before a PS4 is released. I'm thinking 8-9 million tops. This game is coming upon the last remaining time of the consoles' lifetimes and in the age of shooters. In either case, there's as much proof as this game having legs as there is of the contrary. 

I really doubt GT5 will be able to outsell GT3. Keep in mind that GT3 was heavily bundled by the PS2 which is why it sold 15 million. If Sony does the same with GT5, then it might pass 15 million. Otherwise, I think it will sell 10-11 million max and will probably be 2nd best selling GT game.

Also, if we don't have another GT game this gen, then the sales will be much better (obviously).

See this was the type of answer i was looking for, he provides amazingly spot on proof which i hadn't taken into consideration, thanks mate.