As a strict matter of law, the speed limit is the maximum speed at which you may legally drive. There are minimum speeds, which are typically set by statute. Almost no one knows what they are because they virtually never become an issue (I believe King is correct as it relates to interstate highways).
As a practical matter, pretty much everywhere I've ever driven, with no traffic, driving at the speed limit in anything but the far right lane or second to right lane is going get you passed, often angrily.
On southern California freeways (whether they be interstate or state highways), most of which are subject to a 65mph limit, cops aren't going to pay any attention to you if you're driving up to 75mph. It's only when you get to 80mph or if you're weaving in and out of traffic that you're going to draw attention from cops. When I was in Alaska a few weeks ago, we spent a lot of time on the Seward Highway, which is a state highway that mostly has one lane in each direction (and occasional passing lanes). The posted limit was 65mph. Most folks were doing between 65-70mph. There were, of course, a large number of RVs and trucks that were driving as slow as 50mph, and not many folks driving above 70mph. You just have to figure out the accepted practices of where you're driving, but you're ALWAYS subject to being ticketed if you're driving over the posted limit even if you're going with the flow.