At the beginnig of spec/spec_helper.rb:

require 'simplecov'
SimpleCov.start do
  add_filter '/spec/'
  add_filter '/config/'
  add_filter '/lib/'
  add_filter '/vendor/'

  add_group 'Controllers', 'app/controllers'
  add_group 'Models', 'app/models'
  add_group 'Helpers', 'app/helpers'
  add_group 'Mailers', 'app/mailers'
  add_group 'Views', 'app/views'
end if ENV["COVERAGE"]


And add rake task spec:coverage to lib/tasks/coverage.rake:

namespace :spec do
  desc "Create rspec coverage"
  task :coverage do
    ENV['COVERAGE'] = 'true'
    Rake::Task["spec"].execute
  end
end