Tires questions

Hi, eversince I have a 17" rims with high profile tires on my car, the brand new tires seems to wear out faster than stock, specially in the front eventhough I rotate the tires every 10k miles. The tires is Yokohama with the Z rating (120mph).

My question is should I follow the default air pressure for tire from Honda? or fill it up to the max according to the Yokohama tires?

I think the default air is 35PSI or less, but I've tried that and the tires looks as if they were flat... I dun think it would b good for the tires if they are too flat... So I've been pumping 40PSI into the tires according to the tires spec.

I'm also seeing mini cracks all around the tires. Not sure why, I usually drive only at 70-80mph on freeway, besides, the tires is only about 2-3 years old... have like 30k miles on it. Just wierd.

Is it shoulder wear? Is the wear even all across the tread? As fa4r cracking goes, what is the climate in your area like? Have you had you allignment checked latley?

Z rated tires are not 120. v is 135. Z is more like 150.

Check you're alignment, I recently went through $600 worth of tires before it occured to me to check it out. Especially if wear is uneven. Do you use tire conditioner spray, that might help with the cracks.

As far as psi:
"A tire's maximum inflation pressure is the highest "cold" inflation pressure that the tire is designed to contain. However the tire's maximum inflation pressure should only be used when called for on the vehicle's tire placard or in the vehicle's owners manual. " ~tirerack.com

On my honda(90 accord) if you open the driver's door and look in the doorframe there is a little sticker with a table on what the recommended psi for the front and rear should be. That is what I would set them at. However, if your tires come with a recommended air pressure(not just max.) if it were higher then the car's recommended, I would go with that so that the tires have enough air to support their shape and frame.
Underinflation will cause them to wear faster, but overinflation wont help either.

And speed ratings:
P - 93 MPH
Q - 99 MPH
S - 112 MPH
T - 118 MPH
U - 124 MPH
H - 130 MPH
V - 149 MPH
W - 168 MPH
Y - 186 MPH
Z - 149+ MPH ("When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of new vehicles that have extremely high top-speed capabilities.") ~tirerack again.

what about the z rated tire. it is soft rubber and i was told that anything above a v reated tire only lasts about 20k to 30k.