I believe that what your saying is that you can connect to your router at 11Mbps and your neighbours at 48Mbps. If this is the case then it has to be a setting within the router since all other hardware is the same.
Two possibilites, your own router is not 802.11g and so works at a lower speed, or there is a setting that limits the bandwidth of the connection. The third option, that I've just added is a bit of options 1 and 2.
However, you should remember that your sharing a 0.5Mbps connection, so the extra speed doesn't really matter since even at the lowest speed it is 22 times faster than your Internet connection.