It seems like Speedgun isn't in the npm registry anymore.
npm install -g speedgun
npm ERR! Darwin 14.3.0
npm ERR! argv "node" "/usr/local/bin/npm" "install" "-g" "speedgun"
npm ERR! node v0.12.5
npm ERR! npm v2.11.2
npm ERR! code E404
npm ERR! 404 Not Found: speedgun
npm ERR! 404
npm ERR! 404 'speedgun' is not in the npm registry.
npm ERR! 404 You should bug the author to publish it (or use the name yourself!)
npm ERR! 404
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.
It seems like Speedgun isn't in the npm registry anymore.