Here's a patch that displays floats to 8 significant digits and finds the range of such numbers that have the same value. I didn't know how to handle zero so I just ignored it. I'm sure my algorithm for finding the range is naive, overly complicated, and buggy so you'd be wise to check the values before using them for anything important.
float inspector.pd

Note that in this snapshot, the truncated number that Pd displays is not in the range of equivalent values, so the value of this number will change if you, for instance, enter all 8 digits into a message, save the patch, and then reload the patch. This will be true even though the displayed value does not change.