I would be using Fuel w/ Uri::create('controller/method') and wanting to turn that into an https URL. Possible? I would need links and such to work in development and production environs, so my practice is to let the Uri handle the full URL string.
If the entire application moves from http in dev to https in production, just modify your base_url in the config accordingly, and all URL's are generated with the correct scheme.
I personally don't use a base_url, I define '/' as base_url, so all URL's generated are relative to the website's docroot. It also saves you this headache, all links follow the scheme used to fetch the page. It also allows you to have one site that is accessable via both http and https.
If you need to generate specific URL's (for example http for the entire site, but https for login pages), you need to manually craft them like the example in my previous post.