I've read some things that indicated that an 18.4x42 tire would be the same height as a 20.8x38 tire. That may not have been the right two tires that I'm remembering but I'm wondering if someone could enlighten me on how those numbers worked. Is the 42 or the 38 the diameter of the rim? is the 18.4 or the 20.8 the width of the rim?

The thing that got me thinking about this was seeing a 4WD 280 horse tractor that had 14.9x46 tires on it. How would that compare in height? Can a tire that narrow really put 280 horsepower to the ground? Thanks.