Tires: Tire pressure for increased load rated tire, tire placard, gross axle weight

Question
We have a service van, 2006 Ford E150, at about 7,100 GVW due to tools and materials. It came with Michelin 235/70R16 XL rated tires. Due to the weight we went with an E load rating Kumho 225/75R16 replacement tire. After looking at many charts, we are still not sure of a recommended inflation pressure for these tires and vehicle weight. We know the 80 psi maximum inflation would be too high. This is what the installer put in the tires "just to be sure". We would like a more technically founded recommendation for safety and wear for these tires. Thank you.

Answer
Jerry,

The first step in this process is to look at the vehicle tire placard.  That becomes the starting point.  The vehicle tire placard on Ford trucks is usually located on the doorpost.  Not only is the original tire size and pressure important, but in this case the GAWR's become important.

Obviously I can't condone loading a truck to more than the GAWR's.  But in an effort to assist you - and since it is already done - I'll walk you through how to do this.

1)  Locate the placard and record what the original tire size was and the pressure listed.  Also record the GAWR's (Gross Axle Weight Rating).

2)  Weigh the truck fully loaded - even with stuff you don't ordinarily carry.  You want the worst case!  Weigh the truck by wheel position if possible, by at a minimum by axle.

3)  Compare the placard GAWR's with the load carrying capacity of the tires at the pressure indicated on the placard.  (It should be more!)  Call this "Reserve Capacity".

4)   Take the actual weights recorded and add the reserve capacity by axle.  

5)  Look up the load carrying capacity of the new tires and find the pressure that corresponds to the actual weights + reserve.  Round up.

I realize that is a lot of work, but that is the technically sound way of doing it.

But just in case you find this much more work than you are willing to do, I'll make some estimates.

According to Tire Guides, a 2006 Ford E-150 originally came with P235/70R16 XL's inflated to 41 psi with a GVW of either 6700# or 7000#.  At 41 psi, a P235/70R16 XL has a load carrying capacity of 2149#. but because this is a truck, the capacity is derated 10% to 1954#.

Choosing the lower GVW, and assuming that 100% of the extra weight is on the rear axle, then the actual weight increase on the rear axle is 400# - or 200# per tire.  That means the tire needs to be inflated to carry 2154# - or 60 psi.

I would recommend you validate this by checking the weights.  If any of these weights is larger, then the tire could be overloaded and that sometimes has bad results.