<c-x><c-o>  -  auto completing ruby eg pu.. <c-x><c-o> shows "puts"
 <c-x><c-u>  -  auto completing rails e.g  ren.. <c-x><c-u> shows "render"

.....

set omnifunc=rubycomplete#Complete
let g:rubycomplete_buffer_loading = 1
let g:rubycomplete_classes_in_global=1
let g:rubycomplete_rails = 1

......

"perform all your insert completion using the "Tab" key.
"let SuperTab decide which completion mode to use and should play with OmniCompletion
"let g:SuperTabMappingTabLiteral = '<c-tab>' " insert true TAB
let g:SuperTabDefaultCompletionType = "context"
let g:SuperTabContextDefaultCompletionType = "<c-x><c-u>"
 let g:SuperTabCompletionContexts = ['s:ContextText', 's:ContextDiscover']
 let g:SuperTabCompletionContexts = ['s:ContextDiscover']
 let g:SuperTabContextTextOmniPrecedence = ['&omnifunc', '&completefunc']
let g:SuperTabContextDiscoverDiscovery =  ["&completefunc:<c-x><c-u>", "&omnifunc:<c-x><c-o>"]
" let g:SuperTabMappingForword = '<c-space>' " replace default <Tab>
let g:SuperTabLongestEnhanced = 1 " when you have 'longest' in completeopt, enhanced longest.
let g:SuperTabLongestHighlight = 0 " highlight first one in popup menu let you hit <enter>.

Rails 3.2.8
Ruby 1.9.2p320 and tried 1.8.7
Vim 7.3.429 (ubuntu vim-nox package) complied with ruby 1.8.7
Rbenv, Rbenv-build, Rbenv-bundle

let g:rubycomplete_buffer_loading = 1
let g:rubycomplete_rails = 1