Problems during executing rails application. Version Problems

I have some problems running my ruby on rails app properly.

My app normaly runs with ruby 1.9.3 but I got lots of problems with utf-8 and ascii, so I updated ruby to 2.0.0

But now when I run my app with

 rails s

The app starts with ruby 2.0.0 but I haven't the writing permitions which were needed so I got lots of access errors.

If i run

sudo rails s

The app starts with ruby 1.9.3 ....

What do I have to do that the app starts with 2.0.0 when i run

sudo rails s