They are not synonyms. To overstate is to exaggerate. It is deliberate. To overestimate is to make an honest effort to estimate something but to get it wrong. I overestimated how long it would take to drive there. It took only a half hour.

1) Someone makes an overstatement in order to persuade somebody else. "Hosting the Olympics doesn't make economic sense. The costs are typically larger than expected, the infrastructure needed for a big sporting event isn't the same as the infrastructure needed for daily life, and the economic benefits are typically overstated." "Turmeric is tasty and it won't hurt you, but health claims for turmeric are wildly overstated." 2) An estimate is usually a sincere attempt to judge or predict something. We often make estimates ourselves, and we try to make them accurately. An overestimate is just an estimate that was too high. "We have a lot of Halloween candy left over. We overestimated how many Trick-or-Treaters there were going to be this year." "When I started to pack for the trip, I realized I had overestimated how much I could pack in my suitcase. I couldn't take everything I wanted."

Overstatement - is an exaggerated comment. Overestimate - is giving someone or something too much value.

To overstate something to to make a point strongly verbally or in writing. It can be an exaggeration even. To overestimate is to think the extent or what it takes to do something is more than it is. Overstating is conveyed in an action while overestimating is more of a mental perspective on an idea.

I'd love to know that too. As I understand this, "overstate" implies "to talk..." whereas "overestimate" implies "to think...". То есть, overstate могут газеты, спикеры и пр., кто пишет и говорит, в своих суждениях. А overestimate может каждый в своих мыслях. Еще можно добавить сюда же overrated :) movies, or books, or somebody's deeds can be overrated.