alternator belt dimensions

[Snoopy]

Today I broke 2 things: another alternator belt and my "driving on
battery" record. The belt was blown away when cruising @130kmh (same
situation as last time), and I had 45km remaining. Turned off
everything and managed to get home with 11.4 volts left.

Anyway, I decided to choose my belt more carefully this time, and
noticed that original manual (and some shops) suggest 11,2x820 belt,
while family album has some doubts about it. It shows "11,2X820MM", and
on the next text line "to be used for:9.5X825MM". What's the secret
here?


-- 
   Snoopy
'90 100 Avant NF 2.3