Maybe I'm not seeing this right, but have a look at this page.
http://groups.msn.com/OnePieceMangav-3/387.msnw?action=ShowPhoto&PhotoID=8203
When Luffy starts up Gear 2nd, is it just me, or does it look like the rubber is going from his legs to his upper body?
Perhaps Luffy CAN generate rubber… any other paramecium user can generate extras of their power:
Robin: Can bloom any part onto anything.
Mr. 3: Can make large amounts of Wax
Mr. 5: Can use any part of his body as a bomb
Bellamy Hyena: Can create springs from any part of his body (Though he was kinda stupid and only used his legs)
And so forth.
So what's stopping Luffy from making more rubber?
The steam coming from Luffy's mouth and body could be a byproduct of his rubber production; that is, his body is is burning fat into rubber. The problem is, none of the other paramecium users lose weight or mass when they use their powers; on the contrary, they gain mass. Did Luffy gain mass when he learned Gear second, or did he simply translocate mass?