I believe their meaning are similar,although there is this one difference I can think of.
We only use ''tell'' to mean instruct or inform while ''say'' can be used for any kind of talking.
There had been a sentence "Açıkça söyle". I think I translated "Tell it clearly" and was corrected to "Say it clearly". Now, remembering that I wrote "You did not say it cleary" and had been corrected to "You did not tell it clearly". I am struggeling with that. Can anybody clarify please.