I used to have a '66 Chyrsler Newport (heavy car) and it would sit sometimes for over a year without being moved. Never had any issues with flat spots.

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R