That's good for performance, but bad for testing purposes. Different kinds of files compress to different degrees, making it impossible to compare the results of different downloads and different modems. When you download a file from across the Internet, you're adding extra variables, such as congestion between the ISP and the rest of the Internet. Your ISP probably has an FTP site you can download files from. You should also use an efficient downloading program, such as an FTP program, or a plain old terminal program that supports Zmodem.
Web browsers are absolutely terrible for measuring performance.
The HTTP protocol is inefficient, and the browser has to download information, write it do a disk cache, and format the information for the screen. To download a web page with a block of text and ten unique pictures, the web browser must open and close 11 separate connections to the web server. Unfortunately, no, because Netscape and Microsoft's browsers sometimes report inaccurate download rates. A long download can pause for seconds or even minutes without affecting the reported download rate. Stick with an FTP program.