I'd recommend SimpleCov for this.
Here's a nice starting configuration for it to put in your spec_helper.rb:
SimpleCov.start do
add_filter '/test/'
add_filter '/config/'
add_filter '/vendor/'
add_group 'Controllers', 'app/controllers'
add_group 'Models', 'app/models'
add_group 'Helpers', 'app/helpers'
add_group 'Mailers', 'app/mailers'
end
# OPTIONAL
# This outputs the report to your public folder
# You will want to add this to .gitignore
SimpleCov.coverage_dir 'public/coverage'
This makes it ignore files in your test, config and vendor folders and groups controllers, models, helpers and mailers under their own tabs in the HTML report.