Anything over the posted speed limit is speeding (including 63 in a 61). You can also be speeding if you are below the speed limit but still proceeding too fast for conditions.
You are also ONLY allowed to use the left lane to pass slower traffic. You must then move right, regardless of your speed. See RCW 46.61.100(2): "Upon all roadways having two or more lanes for traffic moving in the same direction, all vehicles shall be driven in the right-hand lane then available for traffic, except (a) when overtaking and passing another vehicle proceeding in the same direction, (b) when traveling at a speed greater than the traffic flow, (c) when moving left to allow traffic to merge, or (d) when preparing for a left turn at an intersection, exit, or into a private road or driveway when such left turn is legally permitted."
On the facts you stated the only factual argument you have is that you were going faster than the flow of traffic and were therefore not obligated to pull right and let people pass. The good news for you is that most traffic tickets are won on technical defenses, not the facts. Get a traffic ticket lawyer to help you, slow down and keep right unless passing.
Mr. Lawrence provided a good explanation, so I won't repeat that. The short answer is that the speed limit is the maximum speed at which you may legally travel. Enforcement of that speed limit is the job of law enforcement, not of any other driver. Slower traffic is to stay to the right, and the left lane is for passing. These requirements are relational - they apply no matter what speed you or other vehicles are traveling. If you are moving slower than other traffic, then you need to stay to the right. With all of that said, you might contest this ticket. An attorney could have several arguments to make in defending you.
Saying that you sped up because the police officer behind you was tailgating your car probably isn't going to get you out of the ticket. Talk with an attorney who handles traffic infractions and let them look at the case to see what will get you out of the ticket. Even if there were nothing wrong with your citation, an experienced attorney may be able to get the infraction amended to a non-moving offense for you.
Craig Cahoon The Cahoon Law Office, PLLC 206-795-1779 email@example.com