Car odometers (and speedometers) aren't all that precise. They're designed to keep track of long distances, not measure 1 mile exactly.

If you really want to measure a mile accurately, get a measuring wheel and some spray paint and mark it off on a trail somewhere, or find a running track that is a known distance (though don't expect to be able to ride your bike around a running track without getting yelled at).

Personally, I'd just set the cycle computer as suggested in the manual and go with it. It may be slightly off, but I don't see what difference it really makes beyond comparing your ride with another cyclist. (It's always fun to compare miles after a long ride, just to see who's computers are WAY off though).