The FCC regulates that the analog throughput of all 56K modems be limited to a maximum of 53.333kbps. As a result, you will never be able to achieve a connection speed that is any higher than 53.333kbps. If your operating system reports connections in excess of 53.333kbps, it is simply an error on your operating systems behalf.
Connection speeds depend on the quality of the phone line they are connected to and the type of hardware and software used. In order to ensure the most maximum speed obtainable, one should make sure they are utilizing the latest modem drivers for their hardware configuration and a noise free phone line with clean terminals.