mull over vs speculate

mull over and speculate both are verbs.

Word	Noun	Adjective	Verb	Adverb
mull over	No	No	Yes	No
speculate	No	No	Yes	No

As verbs, mull over and speculate are synonyms defined as:

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

mull over (verb)	speculate (verb)
reflect deeply on a subject	reflect deeply on a subject
	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