The question is what kind of tires you are talking about.
FIM tires, I would always use a 140/80-18 because the knobs are only 13mm high and the 120/80 is very small with less contact surface to the ground.
Motocross tires, I've used one them the Michelin S12 in 140/80 and this tire is a fat pig. I would never use it again. You always have to keep in mind the differences of tires. Also there is a different way to measure, FIM tires are measured all over on the outside of the knobs. Motocross tires are sometimes measured on the outer wall and not the knobs.
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