Originally raised in this topic, the reliability of the various speedcheckers available on the Internet is often questioned. Different customers have managed to obtain varying speeds which, in one case, even exceeded their advertised connection speed!
acarr provided a very good explanation of how the speed tests actually work:
Q Speed test X showed one speed, but speed text Y showed another. Which one is correct?
There is a simple answer to that, and a long winded one.
Simple: Both and neither.
These speed tests measure the time it takes to transfer a pre-determined filesize (even if it is random, a record it kept of the exact size).
Based on the time it takes to transfer the file, a small mathematical equation can calculate the speed at which it was downloaded.
This speed is based on the time it takes to transfer the data between yourself, and the server from which you are downloading.
Because the PlusNet server, and the other server from which you downloaded are physically not the same machine, and are not located on the same network, different transfer rates may be seen between the different systems.
Speed tests are rather flawed for several reasons.
By feeding it the filesize, it will calculate the speed.
The problem with this, some browsers can be slow to parse the page, and thus record a later start time than they should, resulting in speeds faster than actually achieved and in some cases, marked as faster than your connection would allow.
These tests only point out the speed between your computer, and another computer on the Internet, and nothing more.
There are millions of other machines there that you could download from, yet you receive slower or faster speeds from.
The only good they do, is help to diagnose issues at times, and can also help calculate average speeds.
Speeds are only as fast as the slowest link in the chain.
One simple way to monitor your speed while downloading is to use programs like RxTx and Netstat Live, as suggested by gingernutter and chamisra, although this won't test your connection speed but only tell you the speed at which you are downloading/uploading while a file is being transferred.
It's a very small program that you can leave running in the background so you can build up statistics of how much data traffic you have been using (by right clicking on the screen).
So, in conclusion, the speed tests on the Internet are not entirely accurate but, by comparing a large number of results, they can give a good idea of your connection speed.