Treadwear is a comparative rating based on wear rate of the the tire in field testing following a government specified course. For example, a tire grade of 150 wears 1.5 times as long as a tire graded 100. Actual performance of the tire can vary significantly depending on conditions, driving habits, care, road characteristics, and climate.
In other words the tires treadwear is merely a guide as to how long it should last under "normal" driving conditions.
Truth be told, it really has very minor significance in the real world, considering that there are so many variables that affect treadwear, the two most important factors being driving style and alignment. :-\
Despite what the treadwear rating on your Yokohama's claims, they will probably separate a belt before they reach the expected miles. I have had TWO sets of Yokohama's in the past 5 years and BOTH started busting the belts - INSIDE the tread. I have driven on 20-30 sets of tires in my 22 years of driving and NEVER had belts separate. A separating belt shows itself as a big bulge in the tire, sometimes on the sidewall, sometimes on the tread. The last set, Yokohama T4's, I called and e-mailed Yokohama about a manufacturing defect but they would not honor it unless I could prove I did NOT hit something to cause the belt separation. How could I prove this? I have no idea. I hassled them for about four more calls and e-mails but they steadfastly refused ANY type of warranty help. So I let them know I would let all my family, friends and car enthusiats know my disappointment with my Yokohama tires and with Yokohama customer service.
On the flip side, I replaced the Yokos with Bridgestones and have had ZERO problems. Plus, Bridgestones manufacturing defect warranty is for the LIFE of the tire (Firestone and Dayton, also). Most manufacturer's are PRO-rated on THEIR defects. You wear the tire half way, the tread just flies off the tire, they will only give half credit towards the replacement - IF you can PROVE it was not the road or your fault.
Sorry for the long note - just wanted to say, NEVER BY YOKOHAMAS.