oh, and yes...always build in more clearance. the only time that number means anything really, is when you are driving on perfectly level surface under a perfectly measured obstruction.
Any out of level situation at all will change it...
I remember seeing a picture once of a 5th wheel firmly wedged under a drive through overhang at a hotel...with the sign stating the height of the obstruction clearly in the photo...which was above the height of the RV. the photo also clearly showed that the canopy was at the crest of an uphill driveway
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