this speed's (8 mb,16mb) showing to customer only they want to check speedtest via speedtest.net web page or another one.normally they used 1200kbps or 2400kbps sustained.
we don't want to maintain higher speed to customer , all dsl user's will be use same internet pool but we should save their sustained speed.
You could probably use a layer7 match to set speedtest.net connections to a higher speed if that is what your trying to accomplish. I would guess that your not really wanting to do that though.
I would supposed that your trying to conserve bandwidth yet give the experience of high speed.
This can be done through identifying traffic types and working with these types. Burst web traffic. Don't burst streaming traffic like netflix. Netflix in particular doesn't work well with a burst, when it initializes with a burst it wants to maintain that speed for the entire video. Its better to begin and maintain a Netflix connection at 2Mbps then to begin at 10Mbsp and slow down to 2Mbps. However if you load a web page at 10Mbps it could be all loaded in the burst and the user will feel snappy speed.
If you want to do this well then check these guys out: http://www.proceranetworks.com/
but you may have to take out a loan to do it. If you want to be cheap and get close enough then your going to have to get really good at traffic identification and QoS management.
If you don't really want traffic to be managed to that degree then simple bursts may work for you.