Excellent link Jim.

This tire issue is kinda simple .. 

If it's a LT tire the max load capacity is clearly identified.

If it's a p-metirc or passenger car tire, the max load rating needs to 
be de-rated 1.10 as required by the DOT.

Personal opinion: I would never run a P-Metirc tire on a Westy.

Remember these tires are being used on a vehicle that carries a fair 
amount of weight and which is sometimes under harsher than normal conditions.

The other obvious component,

1) What is the weight of your vehicle under full load camp/adventure ready 
conditions. 

- I have seen Westy's loaded for a long distance trip that actually weighed 
more than their max load capacity.

There you have it ... two simple criteria.

Tire load capacity vs actual loaded weight.

What is the load rating on your tire and what is the Max your vehicle could
weigh if loaded to capacity (not beyond capacity).

If a tire supplier suggests using a tire that doesn't seem to meet that criteria, 
contact the Tire Manufacturer's customer support team and ask them if that 
tire is suitable for the actual max loaded weight of your vehicle. 

If it is not, they will clearly tell you they do not recommend  that tire for that 
application and will tell you the warranty would be voided.

The tire is either properly rated to carry the weight of your loaded vehicle or 
it isn't.

Any tire sellers opinion that differs from the above is irrelevant unless you are 
willing to put yourself and your passengers at additional unnecessary risk in 
order to save a few $$$ or have a cooler lower profile tire.

The choice is yours.

larry
roadhaus.com

Date:    Sun, 16 Oct 2011 20:56:38 -0700
From:    Jim Arnott <jrasite@EONI.COM>
Subject: Re: de-rating tires? - was tire circumference -

On Oct 16, 2011, at 5:47 PM, Rob wrote:

> Why are they de-rated?

You asked for it:

The rule is clarified here....

http://www.nhtsa.gov/cars/rules/rulings/UpgradeTire/Final/Index.html

49 CFR Part 571
Docket No. NHTSA-03-15400
RIN 2127-AI54
Federal Motor Vehicle Safety Standards; Tires

On Oct 16, 2011, at 11:00 PM, Automatic digest processor wrote:

> Date:    Sun, 16 Oct 2011 20:56:38 -0700
> From:    Jim Arnott <jrasite@EONI.COM>
> Subject: Re: de-rating tires? - was tire circumference -
> 
> On Oct 16, 2011, at 5:47 PM, Rob wrote:
> 
>> Why are they de-rated?
> 
> 
> You asked for it:
> 
> The rule is clarified here....
> 
> http://www.nhtsa.gov/cars/rules/rulings/UpgradeTire/Final/Index.html
> 
> 49 CFR Part 571
> Docket No. NHTSA-03-15400
> RIN 2127-AI54
> Federal Motor Vehicle Safety Standards; Tires