Unless you have a Docsis 3.0 modem, it is physically impossible for you to get 40-60Mbps. The calculation code is wrong somewhere.
Docsis 2.0 (North American annex-B) uses a single 6Mhz channel, with the highest available encoding being QAM-256 (8 bits per sample). The standardized sample rate is 5.36Msps, so *=8 gives the true peak throughput of 42.88 Mbps, of which ~38Mbps is usable data.
I see the same thing when starting a batch download with DownThemAll, which reports 6+ MBps regularly when starting, but it is simply inaccurate.
(That said, I am very happy with Comcast's "Blast" tier, which regularly gives me 20-30Mbps sustained rates as long as multiple connections are in use, like DownThemAll does. For whatever reason a single stream peaks out around 16Mbps sustained.)
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Gory details on 42.88 vs ~38Mbps usable, if anyone cares:
The standard specifies Reed-Solomon FEC (Forward Error Correction), which mean 6 samples per 128 are unavailable (-4.7%). The MPEG-2 transport stream structure further has a 4 byte header per 188 bytes (-2.1%), Ethernet eats another 18/1500 (-1.2%), IP headers and Docsis map traffic account for about -3% more overhead. It all adds up to ~11%, meaning 42.88*0.89, or ~38.16Mbps -- assuming you are the only person on that downstream, because there is other per-node chatter.
(EuroDOCSIS annex-A spec, btw, uses an 8-Mhz channel with 6.95Msps and correspondingly higher bandwidth.)
