Simple speed logging used for graphing variation. Log output is tab-delimited, which is very easy to put into a spreadsheet and turn into a graph. Logrotate is used to e-mail results daily.

speedtest.cron

# file: /etc/cron.d/speedtest

  *  *  *  *  * root      /usr/local/bin/speedtest 2>/dev/null >>/var/log/speedtest.log

speedtest.logrotate

# file: /etc/logrotate.d/speedtest

/var/log/speedtest.log {
    missingok
    notifempty
    daily
    rotate 30
    create 0644 root root
    prerotate
        echo "Timestamp		Secs	Mbit/s" | cat - /var/log/speedtest.log | mail -s 'Speed test results' myself@example.com
    endscript
}

speedtest.sh

#!/bin/bash
# file: /usr/local/bin/speedtest

set -e

pgrep '^nc$' >/dev/null && pkill '^nc$'

DATE=`date '+%Y-%m-%d %H:%M:%S'`
DURATION=`/usr/bin/time -f '%e' nc speedtest.mydomain.com 8888 2>&1 >/dev/zero`

# Size is assumed to be 10MB worth of \0's
RATE=`echo "scale=2; 10/$DURATION*8" | bc -l`

echo "Timestamp		Secs	Mbit/s" >/dev/fd/2
echo "$DATE	$DURATION	$RATE"