I try to use tabs instead spaces in emacs with ruby-mode, but I can't do it. There is my fragment from my emacs dot-file
(setq-default indent-tabs-mode t)
(setq-default tab-width 2)
I don't write Ruby code, so don't know about the ruby-mode in particular. But it seem that you need to wrap your setting by a hook to the ruby-mode. How about trying this?
(add-hook 'ruby-mode-hook (lambda () (setq indent-tabs-mode) (setq tab-width 2)))