speculate vs excogitate

speculate and excogitate both are verbs.

Word	Noun	Adjective	Verb	Adverb
speculate	No	No	Yes	No
excogitate	No	No	Yes	No

As verbs, speculate and excogitate are synonyms defined as:

Other synonyms of speculate include chew over, contemplate, meditate, mull, mull over, muse, ponder, reflect, ruminate, think over.

speculate (verb)	excogitate (verb)
reflect deeply on a subject	reflect deeply on a subject
to believe especially on uncertain or tentative grounds	come up with (an idea, plan, explanation, theory, or principle) after a mental effort
talk over conjecturally, or review in an idle or casual way and with an element of doubt or without sufficient reason to reach a conclusion
invest at a risk