A truer gauge of network speed

"Ciaran makes a good point in his comment; you have to know what the test is measuring, and this is not clear with a proprietary bandwidth tester.

Downloading a large file will give you a good idea of your TCP throughput, but this measurement will be very sensitive to your latency (Ping time) to the server, and may be affected by your TCP configuration settings (critically your TCP receive window).

Tools like IPerf and ixChariot are used by networking professionals , but these are not really suitable for casual use (IPerf does not work easily behind NAT for example). To gauge the ultimate capacity of your pipe you could flood your connection with a uni-directional stream of UDP datagrams using one of the above tools.

Online bandwidth tools use some tricks to try and push up the throughput beyond the performance of a single TCP connection. I find that speedtest.net generally gives you an average download speed ~30-40% higher than what you would measure by timing the download of a large file. I have confirmed this by timing downloads of the files used by speedtest itself.

Using Wireshark, I made the following observations;

The speedtest client downloads two files simaultaneously while measuring bandwidth - this gives a slightly higher aggregate throughput than a single file
The speedtest speedometer reports a high percentile (90-95%) of the per-second instantaneous throughputs - not the long term average

The speedtest GUI suggests that your download of an mp3 would proceed at the measured rate. This is slightly misleading as the measured rate is more representative of the peak throughput achievable on your link."

I would trust the speedtest.net results over the other ones. I've heard from the designer of the speedtest.net site that most other speed testers use faulty and inconsistent methods. He is of course a biased source, but the consistency of the speedtest.net results would also lead me to trust it more than your other sources.