What is the minimum speed in the left hand lane of the interstate?

Most of the time, if you're driving below the speed limit, you are to remain in that right hand lane. Now there are some places where they do place a minimum speed in the left lane which is below the speed limit. On I25, between Castle Rock and Denver, CO, there are signs indicating a minimum speed limit of 55 MPH in the left lane while the actual speed limit of the road is 65. Even in these instances, it's ill advised. If you're not doing the speed limit, you really need to stay in the right lane.

The minimum speed on Interstate Highways is posted at 45 MPH. Vehicles incapable of travelling at least that minimum speed must not travel on the interstate highway system, or they must be granted a special-use permit to travel on them.

The right hand lanes are reserved for slower moving vehicles and the outside (or where they exist, center lanes) are reserved for traffic which is moving at a legal speed faster than the right lane traffic, or are used when 'passing' slower moving vehicles. Most Interstates have a posted maximum speed at which traffic may move, but on the occasion of passing slower moving vehicles may momentarily exceed when making a 'passing' maneuver.

Another Perspective

Actually, in many states the left hand lane is not governed by a "minimum speed". It is to be used only for passing. You cannot coast in that lane and hold up other cars that want to pass. In Massachusetts many motorists monopolize the left lane thinking that they can coast at the posted speed limit and control everyone else's speed. They can't. If you're not passing other, slower moving cars, then you need to pull over to the right lane. The left land is for passing and you can be stopped and cited by police for misusing the left lane.