Wahl states with the 15 wide hyvo that you can run a 15 wide top gear and commonly available 13 wide lower gear. common practice..? bottom has so mant teeth that wide lower gear is not needed...
You can run 13 wide silent chain bottom gears from Yamaha and Arctic Cat. The 13 and 15 wide V4 chains are silent chains. The 17 wide are hyvo and can not use the silent chain 13 and 15 wide gears.