sudo gem install problem

If I do a gem install with no sudo I get a permission denied...

If I do a sudo gem install I get "sudo: gem: command not found"

Im running Ubuntu feidty fawn with the rminstall stack

Any ideas?

Cheers
Giorgio

why does it work without sudo then..
I get all the messages ..

Bulk updating Gem source index for: http://gems.rubyforge.org
ERROR: While executing gem ... (Errno::EACCES)
    Permission denied - /opt/rminstall/ruby/lib/ruby/gems/1.8/cache/
BlueCloth-1.0.0.gem

seems to be working but not enough permission..

any ideas?