I have an ADSL2+ modem and my max download speed tends to be ~170kb/s (1300 kbps), instead my modem stats claim that it should be almost 5x faster:
Downstream Upstream SNR Margin (dB) : 12.5 db 15.0 db Attenuation (dB) : 32.5 db 17.0 db Data Rate : 6784 kbps 480 kbps
I have the same problem with all my computers, therefore I'm pretty sure it's not about TCP settings.
-- edit: --
All the computers are connected via WiFi. I've tried to change the channel and connect a single pc but (even with a cable connection) the download speed remains the same. The computers have different operating systems (Windows XP, 7 and MacOSX 10.8). I've tried at 02:00 A.M. yesterday and the speed was still stuck at 170KB/s!