excogitate vs speculate

excogitate and speculate both are verbs.

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

As verbs, excogitate and speculate are synonyms defined as:

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

excogitate (verb)	speculate (verb)
reflect deeply on a subject	reflect deeply on a subject
come up with (an idea, plan, explanation, theory, or principle) after a mental effort	to believe especially on uncertain or tentative grounds
	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