Advertised speeds are gestimates, and are done in perfect laboratory conditions. For the same reason a 100 Mb LAN will never reach 100 Mb speed. It's all hype.
Here is a pretty reliable link http://www.speakeasy.net/speedtest/ Pick a server close to you. My Road Runner advertised speed is 15 Mb and I get real close.
I have been using Comcast Cable broadband for a few years. For several reasons but mainly to save money. I recently subscribed to the Verizon's 3 MB/s service. I still have my Comcast service connected. Before my 30 day trial on on Verizon is up I wanted to make sure that it is at least close to the speed of Comcast. I have been comparing the 2 services using various online speed testing web sites. I switch the ethernet cable to my desktop system from one modem to the other to compare. The problem is that some of the speed test web sites show Verizon with a faster connection and others with Comcast. I do not understand that. Is there a better testing site that I should try? Thanks.