you also want the width of the tire the same or greater than the width of the rim, you dont want it smaller.
like if your rim is 8x8, you will want to run tires that are NNx8x8, NNx9x8, and so forth.
if your rim is 8x9, you will want to run tires that are NNx9x8, NNx10x8.
tire sizes are tire hieght x tire width x rim hieght
rim sizes are rim hieght x rim width