Tires: Tires; air PSI, gross axle weight, psi max

Question
Hello Barry, I have a motorhome on  a Ford Chas. The GVWR is 12,400 lbs. Mgf recommended GAWR is FRT 4600, and 7810 in the rear. We just weighed out at a truck stop and came out with the following, FRT-3990, Rear 7100, overall-11,150. We always load out evenly. The manufacture recommends Michelin LT225/75R16, which is what we run. Michelin on the tire: calls for 80 psi MAX.
The Motorhome mgf. calls for 65 in the front and 60 in the rear. (cold inflation PSI)
My question is, what is the best economical and safest air pressure to run. We live in Pensacola, Fla. Also by the time I get to a place that has a air compressor with enough psi to check my tires they will never be cold.
I have received several tire pressures from tire people and mgf's, nothing is ever the same. The most common is a 70 and 68 rear.
What say you ?
Thank you for your time.
Don.

Answer
Don,

The GAWR is the Gross Axle Weight Rating.  It is the calculated maximum load that the weakest component on a given axle can sustain.  

An LT225/75R16 at 65 psi has a load carrying capacity of 2335 # single.  That would be 4670 # for the front axle.

At 60 psi the load carrying capacity is 2190 # single (not applicable here) and 1995 # dual.  That's 7980 # for the rear axle.

From an engineering point of view, I like to see a 10% reserve capacity for tires.  That means 4203 front and 7180 rear.  Sounds like the vehicle manufacturer has sized that tires well for the actual weight and I would stay with their recommended inflation.

Obviously the tires you have now are Load Range E, but you only need a Load Range D.  Personally, I like having the extra capacity

Now for your problem with the cold inflation.  Try this:

Measure the inflation pressures cold with your pressure gauge.  If don't own one, get one.  They're cheap.  If the pressures are low, record how much you need to add to each one.  When you get to the air compressor, measure them again and add what they were lacking.

For example:  If the LF was 62 and it's supposed to be 65, that would be 3 psi short.  When you get to the compressor and it measures 66, add 3 to get 69.

BTW, I don't trust the gauges built into the compressor units.  They don't get checked very often (if at all).  So how do you know if your gauge is accurate.  If you don't have access to a master gauge (and very few people do), compare your gauge with other gauges that other folks have.  Usually, a bad gauge will be the "odd man out".

Hope this helps.