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My machine uses 140/90-16 in the rear. What is the deal with using smaller ones, like 120/90-16? I thought the 120-140 number was the number of degrees of tread on the face, but now I don't know...
 

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On a number like yours it goes Section width/aspect ratio - rim diameter

So your section with is 140
aspect Ratio is 90
And your rim diameter is 16
 

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IOW, if you cut the tire in two, you would see it forms an oval shape that is 140mm wide by .9*140 or 126mm high when mounted on the correct width rim.
 

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2015 BMW K1600GTL & 2008 Harley Davidson Dyna Super Glide Custom
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My machine uses 140/90-16 in the rear. What is the deal with using smaller ones, like 120/90-16? I thought the 120-140 number was the number of degrees of tread on the face, but now I don't know...
Its been my experience to use the original size tires or you might have an issue:

Your tire is 140 mm wide, 90% (126mm)as tall as it is wide on a 16" rim. By going to a 120/90 your tire is narrower (120mm) and shorter (108mm) than the original which will affect handling, suspension, and the overall geometry of the components.
 
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