At 07:39 PM 3/8/2011, The Bus Depot wrote...
> > You offer no source for "the 6% safety margin"
> > you say is required .... There's no
> > mention of the additional 6% here:
> > http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=70

>As per U.S. Department of Transportation, Part 571.110, Subpart B

A proper cite would be 49CFR571.110, but I found it anyway.

>S4.2.2.2 When passenger car tires are installed on an MPV, truck, bus,
>or
>trailer, each tire's load rating is reduced by dividing it by 1.10
>before
>determining, under S4.2.2.1, the sum of the maximum load ratings of
>the
>tires fitted to an axle.

So, there's the 9% derating of the "maximum load."

>S4.2.2.3 (a) For vehicles, except trailers with no designated seating
>positions, equipped with passenger car tires, the vehicle normal load
>on the
>tire shall be no greater than 94 percent of the derated load rating at
>the
>vehicle manufacturer's recommended cold inflation pressure for that
>tire.

Maximum load != normal load. Do you always drive your Vanagon loaded to
full weight capacity? The regulations certainly don't expect that to be
the case, and that's not what they apply to.

You forgot the definitions:

"Maximum loaded vehicle weight means the sum of--
     (a) Curb weight;
     (b) Accessory weight;
     (c) Vehicle capacity weight; and
     (d) Production options weight."

"Vehicle normal load on the tire means that load on an individual
tire that is determined by distributing to each axle its share of the
curb weight, accessory weight, and normal occupant weight (distributed
in accordance with Table I) and dividing by 2."

Note that "Vehicle capacity weight" (i.e. total weight capacity) is not
included.

Table I, as applied to the Vanagon:

"2 in front, 1 in second seat"

So, for the rear GAWR, the difference between "maximum" and "normal" is
roughly the difference between 1 person in back, and loading the back
end to full capacity - that's a very significant difference.