I can play pieces at around a 5/6 Level, but my sight-reading is so poor that it usually takes me a great deal of time to even begin to stumble through each measure of a piece. As a result, I end up needing to attempt to learn a few measures each day and build slowly off of that.
If I can play level 5 , then I should be able to sight read level 5.
So a piece you are capable of learning in a reasonable period of time (which is what being able to play a piece of a particular degree of difficulty generally means), you should also be able to play without any practice at all (sightreading)? Ain't gona happen.