Specule is a proper word, a noun, found in dictionaries, unrelated in usage and etymology to the word speculate in general English.
Speculable/ speculeable cannot be used to imply speculatable, though some instances may be found in writing.
Note that the usage of speculable in a sense related to speculate is found essentially in philosophical discourse:
Thomas calls the objects of practical and speculative philosophy, respectively, the operable and the speculable. [emphasis mine]
(Ralph McInerny, Hist. W. Phil. V.II: XIII C - Chap 6, St Thomas Aquinas)
Data speculatable processor having reply architecture US 5966544 A
A microprocessor having a replay architecture with an execution core for performing data speculation in executing an instruction, …
LLVM Commits List:
I'd like Eric to take a quick look and agree that marking debug intrinsics speculatable is sane. (Yes they already are, but that doesn't mean it's sane. I also want Eric to know that 'speculatable' is going to start showing up in his debug info.)
Also, non-speculatable, speculatability, non-speculatability:
The method and apparatus are employed within a microprocessor capable of generating speculative memory accesses instructions. Certain instructions access memory locations containing speculatable information while others access memory locations containing non-secpulatable information. Memory-type values indicating the speculatability or non-speculatability of memory locations are stored within a translation lookaside buffer. (Method and apparatus for handling speculative memory access operations US 5956753)