<p>Where is the best place to add a method to the integer class in Rails?
I'd like to add a <code>to_meters</code> and <code>to_miles</code> methods.</p>

Normally (and logically), integers can't be converted to miles or to meters. It sounds like you may want to create a new class like "Feet" or "inches" that is initialized with an integer, then contains methods like size_in_miles or size_in_meters. For convenience those methods could return decimal or float types, but you might also want to write a miles class or a meters class.
As an alternate method, you might want to create a static method in your new class that would have a signature like this:
```
Float feetToMiles(integer I)
```
that you would call
```
miles = Feet.feetToMiles(5280);
```
and get
```
miles = 1.0
```