(in the US) Unless otherwise posted, 25 mph is commonly the universal speed limit in populated areas of an urbanized environment. Other than that, every state has a maximum speed limit codified in their laws. You may not exceed the maximum speed limit for your state, whether it is posted or not.
The maximum legal speed limit on a rural Interstate highway is the posted speed limit in the United States. Each state in the United States has different posted speed limits for Interstate highways and other highways.