do i have to replace all four tires? had a flat...

[circmand]

Etlsport however

Sorry Circ, I gotta side with Eric on this one.

No logic involved ... just preference.

Well, maybe a little logic:

You drive a vehicle that can apply drive to all 4 wheels. That vehicle is designed (within reason) to do so with tires that match. If the tires don't match then obviously you are causing the drive system to work harder to compensate for the mis-match. You'll either cause a clutch system to slip more or a viscous coupling to slip more. Either way (in our case it's a clutch system) it will cause additional wear ... either clutch wear or additional heat generated by the viscous coupling. The bottom line is that if your tires don't match, it may not be unsafe, it may not damage the vehicle, it WILL cause accelerated wear of certain components.

That's all I got to say about that.

Oh and Tom4bren your arguement fails to take into consideration the vehicle suspunsion. With the independent suspension and the weight of the vehicle all pushing downward the small diference in tire size does not affect the AWD. Its not like a solid suspenion where if you set the VX down on a flat surface the tire that is 2mm smaller in diameter (which means since the tire is attached at the center the tire is only half the diameter difference) the tire would not hover 1 mm above the surface. The suspension would still push the tire doan and contact would be solid. The radius is actual what should be measured. Heck a 1-2 mm difference would probably not be much different than driving on a paved road surface that is crowned.

[tom4bren]

Actually ... NO!!!

On paper, it's all about radius. On the road, it's all about circumference. I know that circumference is directly proportional to radius (high school was a loooooong time ago, but I remember the simple stuff). But, when the rubber meets the road, the tire can no longer be treated as a circle - it's flat where the contact is made. Believe it or not, the radius decreases (mushes down) but the circumference remains constant. A worn tire has less circumference (distance traveled per revolution of the tire) than a new one. Suspension has no bearing on the equation.