Tires: Load Index, nissan xterra, winter tires

Question
QUESTION: Barry,  I have a 2003 Nissan Xterra XE 4WD.  I am buying winter/snow tires.  The OEM size tire is P265/70 R16 at 35 PSI.  I want to go with a 245/75 16, because I think that the wider/lower profile 265 isn't as safe on ice.  I have had winter tires in the stock 265, and thought they had too much float.  My wife's Grand Caravan with all seasons actually handles ice/slush/snow better than the Xterra on winter tires.  The problem is that the winter 245 comes with a load index of 109.  For some reason, my Xterra is rated for a 111.  The reason I say for some reason is that the curb weight of the X is only 4080 LBs.  The GVWR is 5200.  Both GAWRs are 2850.  A 111 rating equals 2403 LBs x4=9612 LBs.  A 109 equals 2271 x4=9084 LBs.  That is only a 528LB difference, or 132LBs per tire.  As I said, my vehicle GVWR is only 5200 LBs!  Even with the 10% reduction for a P metric, that seems like overkill.  My wife's Caravan has a higher GVWR, with a load index of 96.  If I am not mistaken, I believe that there are full-size pickups with a lower load index than my Xterra.  My only thought is that the 9500LB total vehicle plus trailer weight must be considered.  To complicate things further, Xterras have different load indexes from 102-114 for 2WD, 4WD, and different trim packages. I do not tow during winter, and don't haul heavy loads.  I would really like to get these tires. Do you think it would be safe to mount a 109 tire?  If so, is there a formula to determine the correct PSI?

ANSWER: Bryan,

The way in which tires are selected by vehicle manufacturers is very complex.

But you've come across a principle of engineering I like to call "Over-design / Under-utilize".  The fact that Nissan spec'd a larger than minimum load carrying capacity is this in action. This is done for quite a few reasons, but the most important reason is safety.

I would not use anything smaller in load carrying capacity.  To do so increases the risk of a load related tire failure, which sometimes has tragic results.

If you want to read up a bit more on the subject:

http://www.barrystiretech.com/loadtables.html



---------- FOLLOW-UP ----------

QUESTION: Barry,  thanks for your prompt reply.  If you recommend a 111, then I am going to have to either go with the wider 265, which I believe is counterproductive safety wise, or get the 145 in an LT.  How do you feel about an LT?

Answer
Bryan,

LT's are completely different animals and I'd recommend against that move.

But I suspect that a lot of your ideas about how the tires deal with winter conditions may be more a reflection of the particular tires that you had - and not so much about the tire size.  The difference in performance between tires can be enormous.