Getting messy values from Sharp IR sensor

Using Arduino Sensors
4

What is the application?
You should start with the behavior that is needed. Lets suppose this is a small robot that should avoid bumping into a wall.
The rules:

  1. < 1.5v full speed ahead
  2. 1.5v - 2.5v slow down

  3. 2.8v stop and turn

Of course I just made it up. The point is you may not need any conversion depending on the application.
If you really need distance you can use an algebraic equation or a lookup table with interpolation. I prefer lookup tables, which can be setup directly from test measurements. Simple interpolation will improve accuracy if needed.