As far as actual speed vs garauntee speed, it can only be truly tested within the ISPs network. Normally this is tested by uploading a file and downloading a file via FTP to another computer. This other computer must be accessable (see traceroute) without leaving the ISP's network. If you use CLI FTP, turning on bin and hash, it will give you the speed. Files should also be large, like 5MB or so. Online speed test sites use very small files to do the tests, and they are not accurate this way. You need to know your average speed, which requires a larger file. You will notice if you download something, it starts out pretty fast and then slows down to a certain speed. This is why small files should never be used for speed tests.