I have used  3" round convex stick on mirrors on both sides for years
and wouldn't drive without them.  I prefer mounting them on the bottom
of the mirror since I am really checking for cars that are below my
window line, hence looking down.  Just my preference though.  I have
tried smaller round and small rectangular mirrors in the past, but found
them too small.  The three inch lets me see what is there pretty well.
--
Stuart MacMillan
Manager, Case Program
800-909-8244 ext. 8208
Fax: 206-269-6360

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