What is "internet speed" to you? Is it derived as

    distance_flown_by_the_internet
---------------------------------------
time_when_it_lands-time_when_I_threw_it

Or is it the speed of data through your internet interface? If the later, I use the following script as an input to MRTG graph:

open(INP,"</proc/net/dev") || die "Can not open /proc/net/dev: $!";
while (<INP>) {
    s/eth0:(\d)/eth0: $1/;
    @a=split;
    print "$a[1]\n$a[9]\n" if ($a[0]=~s/eth0://);
}
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This, of course, is highly system dependent...