Connection Speed v Bandwidth Test

That's about as good as you will get from a 56k modem. The practical limit is 53k. Your connection speed probably is better than average. Indicative of a clean phone line and reasonable proximity to the phone exchange. You could check your external phone junction box for a test module that frequently is installed. Removal can improve your service slightly. Go to this site and DL a little program called DrTCP. It will allow you to see and modify your settings.

http://www.dslreports.com/front/drtcp.html

The best value for MTU is that just before your packets get fragmented. Use ping to find that value. Open an MSDOS command prompt, and just use ping.

ping -f -n 10 -l 576 host_or_IP

Then reduce 576 by 10 until you no longer get the "packet needs to be fragmented" error message, then increase by 1 until you find the max command line number for ping.
Add 28 more to this (since you specified ping packet size, not including IP/ICMP header of 28 bytes) AND THIS IS YOUR MaxMTU. Typically will be 576 for Win95/98 with DUN 1.3

Receive Window (RWIN) should be larger than the bandwidth delay product (also known as pipe capacity). Note that if either available bandwidth or the delay is doubled, RWIN would double as well.

For example,for 170ms delay on a 56Kbps link, RWIN = 56000 bits/sec * 170 msec = 9520 bits = 1190 bytes (1.2k).

W95/98 RWIN default is 8k / W2000 is 16k.

You can use 'regedit' to search and edit keys specifying RWIN, MaxMTU and/or IPMTU (these are case sensitive).