Wheelbases.
.
Anyway, here is the text from the chart. I'm not sure of it's accuracy, it's just a copy/paste of what I googled...
'59-'60 F-series long bed 120"
'61-'63 F-series long bed 120"
'64 F-series long bed 128"
'65-66 F-series long bed 129"
'66 F-series short bed 115"
'67-'72 F-series long bed 131"
'67-'72 F-series short bed 115"
'69-'72 F-series short bed crew cab 148.3"
''73-'79 F-series short bed crew cab 150.3"
'73-'96 F-series long bed regular cab 133"
'73-'96 F-series short bed regular cab 117"
'78-'96 F-series long bed super cab 155"
'78-'96 F series long bed crew cab 168.4"
''97 F-150 long bed regular cab 138.8"
''97 F-150 long bed super cab 157.4"
'97 F-150 short bed regular cab 120.2"
'97 F-150 short bed super cab 138.8"
'66-'77 Bronco 92"
'78-'96 Bronco (Full size) 104.7"
'84-'90 Bronco II 94"
'83-'97 Ranger short bed 107.9"
'83-'97 Ranger long bed 113.9"
'86-'97 Ranger super cab short bed 125.4"
'91-'97 Explorer two door 101.7"
'91-'97 Explorer four door 111.5"
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