Some details:

Latest versions of Virtualdub and Divx.
Windows 2000 machine with boatloads of RAM and CPU.

Have a 45 minute AVI (17.2 GB) I want to compress to
225-240 MB using Divx.

Have Variable bitrate mode to "1 pass."

Set encoding bitrate to 640 kbps, get 330.9 MB file).
Set encoding bitrate to 480 kbps, get 326.4 MB file).
Set encoding bitrate to 320 kbps, get 325.8 MB file).

I have pretty much left everything else at the defaults.

Can somebody explain this? 640 kbps should give
2700 sec X 80 kBps = 216 MB (plus 10 MB for audio),
but even with a bitrate of 320 kbps, the Virtualdub
info window shows more than 100 kBps while encoding
(the MP3 audio totals about 7 MB). :(

Thank you in advance.