Tcagle

Hi All,
Forgive my lack of engineering prowess, but could someone give me the engineering-101 on why you should (have to) replace all 4 tires at the same time if you only need two tires replaced (on an AWD car)?

Thanks,
Todd

bclexus

It's all about the diameter tolerance of the tires on a AWD vehicle. Since an AWD vehicle uses all four of its wheels (tires) to apply power you can probably imagine how it would present a problem if one or more of the four tires had a significantly different outside diameter, or a different tread design. Some car makers suggest that all four tires should be replaced at the same time to keep all four tires' diameter tolerance at a bare minimum. However, other car makers feel that staying within [say] 4/32" in tread depth (as long as all four tires are of the same brand, size and tread design) is within an acceptable margin. I do not know what Lexus stipulates for their AWD model vehicles...

er34

All 4 wheels are "connected" via the transmission/differentials. When you have wheels spinning at different rates(and this happens under normal condition like during cornering), the differential is what keeps everything working happily. With the different tire sizes, you are essentially making the differential work hard even while driving straight - more wear on the fluid, gears, overheating.

You don't need to replace all 4 if you can ensure the tread depth are the same - TireRack has tire "shaving" services to make your tires the same depth as your existing ones - up to you whether you want to do that or just buy 4 new rubbers.

Tcagle

Thanks for quick replies. Makes sense. Guess I'll spring for 4 new ones.

bclexus

Rotating your tires every 5k to 7k miles should have them wearing equally so this doesn't happen again to you.

Also, if you are buying new tires you should have your alignment checked and corrected.