How about using one of the 4.00-6 foam filled ones meant for a hand truck or wheelbarrow? HF has a knobby "13 inch" one for about $22 which might fit, depending on your axle size. Then never worry about flats again...
I have DR chipper with 410/350-6 tires. There are dry rot cracks and one keeps going flat. Replace or tube? <img src="http://www.tractorbynet.com/forums/attachment.php?attachmentid=465548"/>