Speedtest.net frequently produces impossible results. Why not use Acanac's speedtest instead »www.acanac.ca/speedtest/
Even smarter would be to telnet to the modem (root / admin) and run this command:
cat /*/*/*m_s*
The Maximum Attainable Bit Rate is the modem's estimate of your DS line capacity. It's probably close to 3Mb/s, which is pretty good, considering your very poor Attenuation.
