afpj

Hello all,
I read some older posts about changing more than just one tire at a time on AWD, but there was some waffling between 2 vs all 4 simultaneously. I just replaced 2 original Michelin MXV 4 (65k miles) and replaced them with Bridgstone Alenza's (currently both on the front). The Michelins that were just replaced were due to a crack and exposed belt, but still had 4/32 tread left at least.The rear tires have Michelin MXV 4 that have about 20k miles on it but have alot of tread left. These Michelins were put on by the previous owner just before I got the car at 45k miles. So, this car started life with me with somewhat uneven tread...now should I get the last 2 Michelins replaced to make all 4 equal tire pattern and somewhat even tread depth? 05 RX330 at 65k miles. Thanks for any advice. This is my first AWD car so I don't know about those peculiarities.

Alenza tread depth = 12/32
remaining Michelin = 7/32

Lexmex

There can be issues mixing two different types of tires given that they have different weights and revolutions per minute. I wouldn't want to do that and would stick to the same type of tire.

I have had a situation where I had older Michelin CrossTerrains of the same size mixed in with new CrossTerrains, where I have gone with two different sets on my RX300 for a period of about a month to up to 3 months, including just recently since I got to the U.S. (got 2 rear tires in Mexico and then 2 new front ones here in Florida) and I also did this in late 2005 back in Mexico City. I can't say that I noticed an issue with my transmission, but I did notice that when I floored it, the rear tires would drag (since they had more tread and more rotational mass). However, if you do this long term you can cause some issues with the tranny and transfer cases/rear differential.

If you had the same tire all the way around, even with different treads between the 2 sets of tires, it's still be a judgment call with the mileage you have on them. Being theoretical here, and assuming I was going to keep the tires you have...I'd put the newer tires on the front (they will wear faster due to the weight of the engine) and given how the AWD system works in these vehicles the drag will be less on the rear wheels having the tires with more tread off of them.

However, the problem I have that mixing the Bridgestones is the problem here because you have a different tire weight and revolutions per minute which adds to the issue already of different tread sizes.

afpj

Thanks for the info. I wasn't sure how the AWD came into play. I thought they (the front vs rear axles) might be more independent with fancy viscous coupling and what not. My primary 4wd experience was with a system that you had to shift under 10 mph, locked all 4 wheels in the same rotation. A different sized tire (like a spare) would cause some noticeable scrubbing on dry pavement. I was hoping the lexus and other "modern" awd systems would be more forgiving, but I guess not. Thanks again for your input.
Regards

Boognish

Lexus states that the vehicle has a viscous coupling for the center differential. Basically, the way these things work is as follows:

You've got a clutch pack that is swimming in a fluid. When there is a different speed between the front axle and the rear, the clutches spin at different rates and create friction which heats up the fluid. At higher temperatures, the fluid gets "tackier" causing the clutches to grab and effectively equal the speed of the axles front to back. The system is not perfect, and it will never fully lock, but having said that, if you've got different size tires front to back, the fluid is constantly going to be at an elevated temperature the viscous coupling unit is going to fail prematurely.