Presumptive means based on presumption. It’s often synonymous with probable. Something that is presumptive can be reasonably guessed based on ample evidence.
Presumptuous means (1) going beyond what is proper, or (2) excessively forward. One who is presumptuous presumes too much.
Examples
Presumptuous
It seems cold to disappear to the spare room, presumptuous to sleep in “our” bedroom. [Guardian]
It is somewhat presumptuous of me to offer anything since I only know Christchurch from pictures and descriptions. [quoted in New Zealand Herald]
I think it’s a little presumptuous for anyone to say, “No marriage can be happy without sex” when what they mean is “I could never be happy in a marriage without sex.” [Chicago Sun-Times]
Presumptive
And Donald Trump’s birther-fueled talk can rocket from Internet meme to presumptive presidential candidacy in a matter of days. [Politico]
And what do these presumptive threats tell us about the complex relationship between man and nature? [Forbes]
Toscano, the New York Mets’ annual “Anthem Search” winner in 2008, admitted her presumptive frontrunner status might have cost her support. [New York Daily News]