[ale] way OT - used car buying tips - yea or nea - PT 2

[Sean Kilpatrick]

I am very suspicious of this.  There are significant differences between 
four-wheel drive and "all-wheel drive."  The latter, which most light-duty 
SUVs have today, is for temporary use only on slippery surfaces as the 
drive electronics lock the differentials. That is, the wheels on the left 
side are turning in lock step with the ones on the right. When the diffs 
are locked, then the left/right tire pairs have to be very close to the 
same circumference or the constant drag/slippage almost certainly will 
cause problems. On snow, ice, or mud, not really a problem for small 
differences.

The problem, such as it is, is real for vehicles (think older Land Rovers) 
that have the ability to lock any of the three diffs independently.  At 
that point the tires need to be the same size to avoid problems.

Today almost all "AWD" vehicles are full time front-wheel drive and have 
only two diffs: One for the front and one for the center.  When the center 
one is "engaged" the back wheels turn as one and the front diff also is 
locked. Significant differences in tire circumference will put stress on the 
drive train.

But a thirty-second or two is probably not significant.  The tire on my SUV 
has a current diameter of 28.25 inches (more than 30k miles.)  Suppose I 
replaced it with another used tire with an additional 2/32" of tread.  The 
rolling diameter of the smaller tire would still be 99.78% of the larger.  
If this were a real problem, there would be many more drive train failures 
caused by installing the spare tire and continuing down the interstate.
(And, no, I wasn't willing to haul out the spare tire and measure its 
actual diameter.)

Sean

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On Wednesday, November 28, 2012 05:16:11 pm Ron Frazier (ALE) wrote:
> Re: awd - All wheel drive can be very handy.  I almost bought an AWD
> Mazda Tribute.  However, it had an unpatchable screw AND nail in one
> tire.  They said I had to replace all tires at once to keep them the
> same circumference.  I did some research and it appears to be
> true.  You have to keep the tread depth of all tires within 1 or 2/32
> of an inch or it screws up the drive train and can cause premature
> failure.  I never knew that before
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