* Your owner's manual: This is the best place to find this information.
* The wheels themselves: The bolt pattern is often stamped on the back of the wheel.
* The inside of the hubcap or wheel cover (if present): Sometimes the pattern is marked here as well.
* Your vehicle identification number (VIN): A Ford parts website or a mechanic who specializes in classic Fords might be able to look up your specific wheel information using your VIN.
Without more information about your specific 1966 Ranchero, I can't give you a definitive answer. Common patterns for Fords of that era include 5x4.5" (5 lug bolts on a 4.5-inch diameter circle) and 5x5.5" (5 lug bolts on a 5.5-inch diameter circle), but this is not guaranteed to be correct for your vehicle.
