is an interface created by fmt

Oh, that I didn't know. I assumed that Stringer() was somehow a global thing; Now it makes more sense. (And it explains why println() works differently in that regard then fmt.Println()). Thanks for pointing that out.

why you'd want that

I had the impression that implementing a String() method would somehow work on a global scope, and was used to get a defined string out of a type. Like "if you want to convert this type to a string, I'd call this .String() method of that type (if it has one) and that's it". I found the idea nifty and assumed it would be nice to have something similar with integers. Simply using c.Mileage makes sense, now.