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In your own opinion, does the tire size make a difference on a 125?
I got a 05 yz125 and the previous owner had a 120/80-19 tire on it. Just wondering what everyone thinks is the best size. Should i switch it back to the standard 100/90-19?
Whats the pros and cons of each.
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You’re shooting yourself in the foot with a 120/80-19 on a small bore. You will absolutely notice a difference in power between the standard size and a tire designed with a 450 in mind. Plus, you will also notice an improvement with lean-in entering turns. A bigger/taller tire wants to stand up more it seems, where the standard tire will lean over with ease and arc the turn better. I suggest a Michelin Starcross 5 Medium or Soft 100/90-19. Pirelli 32 or Dunlop Mx3s would be my 2nd choice.
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A YZ125 with a 120 rear on will probably feel like an 85 in anything except hardpack conditions. A 100 for sure, but can get away with a 110 on a quick yz125 or husky/ktm.
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My (used) '05 YZ125 came with a 110/80-19 with some aggressive (low) gearing. I liked it, so I got another one (MX51). Don't think Dunlop offers a 110/80 any longer, so I'll be going with a 100. Ride mostly off-road and hardpack throughout the year with this bike.
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Run a 110, it will stick in ruts and flat corners.
Pcp252... that’s funny that a 120 was designed for a 450 when I ran 120’s before there were race 450’s... try again young buck
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100/90-19 on a 1.85 width rim! Only way to go 
Some of the 250F's can pull the bigger tires/2.15 rim, but not the one-two-fiver's...
