You may have overlooked a fact of tire ratings. Your XL tires reach
their maximum weight carrying rating at 42 psi. Sure they can be
inflated up to 51 max but that doesn't increase their rated carrying
capacity beyond what they had at 42. The extra inflation pressure range
is to allow adjustments for handling in a particular vehicle. So the
pressures you are running significantly exceed the pressures needed by
your tires for your loads.

Incidentally, I run 100 XL and 102 XL tires on 3 of my Vanagons. Mine
also go 51 psi max. I typically inflate to 40 front and 45 rear.  On
trips I may adjust up or down a few psi depending on what I am carrying
and the roads I expect to use.

Mark

Mark Hersh wrote:
> I ran across this paper on the interweb
>
> http://www.dawsengineering.com/linked_files/tirepressureatreducedloads.pdf
>
> that says that a conservative estimate of load supported by
> less-than-maximum inflation is a simple proportion.  A more accurate
> estimate would take into account the stiffness of the sidewall.  More
> conservative is fine with me.
>
> Example, with the Nokian eNTYREs I just put on Monty:
>
> First, I reduced the max load by 10% per some official body's guideline
> when running an XL tire on a van/truck.  So the 1764 lbs max (100T rating)
> on the tire is 1587 at 51 psi.
>
> The Gross Vehicle Weight Rating on the plate on Monty's door jamb is front,
> 2646 lbs, rear 2866 lbs, so I need 1323 lbs supported per tire on the front
> and 1433 on each rear tire.
>
> Those particular weights could be supported by 46 psi in the rear and 42 in
> the front.  So I'm thinking that's the MAX I will ever have to inflate
> these tires.
>
> I did not get much help from Nokian when I asked for a load-inflation
> table.  They asked me what psi's I was interested in, so I told them 36 to
> 51 and they have not yet responded.