how much psi to fill tire?

Ok guys put your thinking caps on.My Wheelin truck at work has rockwell axles,air lockers,tandem axle lockers,dual power steering gearboxes, a i6 turbo diesel and a 30,000lb hydro winch and hydro outriggers that'l lift the tires 4' off the ground.yep it's not a jeep but it can lift almost any jeep out there and if it fits under the boom i'll bring it home from a travel job.my question,Does a compressor need more power to fill a tire with the truck on its wheels rather than in the air?We were tossing this one around at work and came up with some good logic but no real conclussion.See our shop compressor runs 120psi and the tires run 100psi but its dificult to fill the tires to capacity,I think the problem is lack of volume in the holding tank but am not positve.  It seems to reason that psi is psi regardless of the wieght on it,also the closer you get to the max psi of the compressor the slower the tire will fill,still 120psi should fill 100psi right?So I don't really need an answer since the tires get checked every 6 months at inspection time at the shop with a bigger compressor I'm just curious what you guys think and if my logic is sound so i can tell the guys at work whats what.Thanks  for your time.Oh yeah i get this truck stuck more often and into more tight spots than any of my jeeps and it's 42' long and needs a excavater or bulldozer to get it out!!  

its faster to jack it up and air them up than with the weight on them, just fillem to the psi that the sidewall reccommends  
Using the gas your hybrid saves!!

Sounds like the compressor needs more volume for your application.  It's kind of like the cheapie 150 psi cigarette lighter compressor to inflate a tire vs. a garage air compressor the psi output is about the same but the volume is more. ( cfm cubic feet per minute) will make a huge difference.  

I'm going to go with both. With the compressor you have it will be easier to fill with the tires not supporting the weight of the entire vehicle.  
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As soon as they offer a Ranger with a Diesel in America, I'll be the idiot camping out in front of the Ford Dealer!

So far so good,any more ideas?I think i already knew my answer since i've thought about this for more than a year now,just curious what others think.Thnks  

CFM is the rating for a compressor's ability to do work. And CFM is volume at a given pressure. As the pressure increases, the CFM will decrease.

The air moves slower as the pressure differential decreases.

The weight of the vehicle on the tire can cause a slightly higher pressure with the same volume of air in the tire. But that is only when the weight causes the shape of the tire to change and decreases the tire's volume. It is the decreased tire volume that causes the higher pressure, not the weight itself.

A compressor with more CFM at any given PSI will fill the tires quicker regardless of the vehicle's weight on or off the tire. And a higher tank psi, causing a greater pressure differential, will speed the flow of air as long as there is enough volume of air put out by the compressor to keep the tank pressure high.

To see the difference, measure the tire PSI with the vehicle's weight, and then again without the vehicle's weight. The pressure difference will be very small and therfore make very little, if any, difference.  

Yep you got it Giles! The whole weight thing can distract you from the simple truth.I had to think about it for awhile to be sure.Another way to put it is the tire is basically another holding tank, the compreesor will run until the high psi switch triggers.The compressor I have will fill the tires but the time it takes to do it is impractical.Thanks for the info hope you had fun.