AFAIK, tyre flaps are sized accordingly to the inner and other tyre they are to be used with. So you would be looking for 11.00-20 inner tube to go with an 11.00-20 tyre.
BTW, the rim/wheel size is 6.00x20, so the width of the rim where the bead sits is 6 inches wide. The tyre's size is 11.00-20, whereby the width of the contact area is 11 inches.
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Regards,
Hanno
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