How much does my older cable modem affect download speed?

How much speed loss would be expected from a wireless router in the
mix?

I'm thinking of down-tiering to Comcast 1 Mbps service (we do very
little video) would I expect a similar percentage drop?

thanks

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from my post at alt . online-service . comcast
original:
download speed testing

I get around 4.5 to 5.5 Mbps using these:

http://speedtest.comcast.net/
http://www.speakeasy.net/speedtest/

but only get 2.7 to 3 Mbps using this site:
http://www.speedtest.net/index.php

according to Comcast I should be getting 6 Mbps

Anybody test lately and which sites seem more believable?

(should I complain that I don't get 6 Mbps)

note:
upload speed seems more consistent around 1 Mbps
It seems that the chosen server for each test excessively influences
the test

a response:

> Old modem? Post modem model and version numbers
> "IF" your modem supports it check signal power and snr athttp://192.168.100.1/
> If you get the right person when calling Comast they might check your
> signal levels from their end if your modem doesn't support it..

my followup:

SurfBoard 3100 appears to be DOCSIS 1.1

Downstream Value
Signal to Noise Ratio 37 dB
Power Level 0 dBmV
The Downstream Power Level reading is a snapshot taken at the time
this page was requested. Please Reload/Refresh this Page for a new
reading
(this did not change on refresh)

Upstream Value
Power Level 56 dBmV

what is a good reading?