tow vehicle tire air pressure/load quandary

I switched from the OE P rated (35psi on the door) to LT (E rated) Michelins. Asked Michelin what pressure to run the LT's at. They asked what my previous OE tire was (size,make) and what pressure Ford wanted me to run it at (35psi). They then told me what pressure to run the LT's at so they would match the load rating of the original tires on the truck (48psi for the LT Michelins).

Simple. Just match the load rating of the original tires.

As for wheels, AFAIK alloys are stamped with load rating, not air pressure because they can handle any air pressure that tire's are rated for. There are steel trailer tires out there with specified air pressure max's though. Maybe the welds are a weak spot?

Good point on the maximum force a wheel can sustain. Most are pretty high though so no real need to worry.

But I don't understand how a tire manufacturer is going to know what pressure is needed for a given combination of weight and vehicle characteristics. Most tires are made to general specs to fit a range of vehicles and uses. What they will give you on a chart is the maximum rating for a given weight, not knowing the handling capabilities of the vehicle in question. Some good reading here: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=196

Contact the tire manufactuer they have a chart that will tell you what pressure is needed at the weight you are trying to carry.
Unless you are going to run bias tires you can not tell if a tire is overloaded by sidewall buldge. Different manufactures may recommend different pressures at the same load rating. Also there is a fantasy about putting heaver tires with higher pressure rating to carry a bigger load. Remember a tire and rim assembly is a pressure vessel. The rim the tire is mounted on has a pressure rating also.

I am with CapriRacer on this. For the most part I have always used the rating in the door for all my vehicles, including while towing. I always get the max mileage out of my tires and have never had a blowout or any handling issues. Chevy assumes a load when they put those ratings on the door. How do I know this? Because on my Trailblazer and my last 2 Suburbans the rear tires were supposed to have more air on them.

Timeking, you do want tires that have more "wiggle room" on the PSI though. At least 40PSI max. and using a so-called "SUV" tire. It's not an LT (which would be far better! If your TB has the 17" wheels, you can't even get an LT that will fit) but its better than a straight up "P Rated" tire. I have Firestone Destination LEs. They have a softer sidewall then I would like so I run them 3 PSI higher than the door rating. That has worked great. They are 4 years old now and need replacement only because they are developing cracks between the treads. I don't disagree that you should see very little bulge when loaded, but at the same time maxing out a tire's pressure can put less rubber on the road and cause excessive wear in the center of the trad pattern. Honestly, you have to experiment a bit and find the happy medium.

What CapriRacer means by his statment is that if the sidewall of the tire says "Max PSI 35", don't put it 40 PSI because someone said that's what they do. 35 PSI max means 35 PSI max.

CapriRacer: On your wwww you state "But under no circumstances am I advocating the use of inflation pressures higher than what is written on the sidewall (with the exception of tires with 35 psi max. on the sidewall - and there's a long story there!)". What does that mean?

If you will allow me, I'll give some insight into what the vehicle manufacturer does regarding the pressure that is listed on the vehicle tire placard and in the owners manual.

Different vehicle manufacturers use different methods to arrive at the tire pressures, but once they get there, they select springs, shocks, and sway bars based on that pressure.

Then they test. They test the vehicle in all configurations - fully loaded, empty. If the owners manual calls for a different pressure when empty, they will have tested it.The handling tests are quite severe - commonly called limit handling.

So if your vehicle owners manual does NOT call for a different pressure when empty, the vehicle manufacturer probably did not test that condition.

We know that a tire's spring rate changes with pressure - and changing pressure that changes the handling balance - the understeer/oversteer characteristics.

I am of the opinion that the vehicle manufacturer's specifications should be followed because those conditions are known. Anything else is unknown.

Your door post gives the correct air pressure for the trucks GVWR (when using OEM size and type of tire. You would probably benefit from running truck tires IF you can find them in a size that fits your Blazer. Probably not much benefit from just running bigger passenger car tires as the larger foot print will offset any fuel savings form higher air pressure.

Buy tires rated for the weight you need plus whatever cushion you are comfortable with. Air them up to what is on the sidewall, and have a safe trip.

You can buy a higher rated tire and use the higher air pressure when pulling and when your not pulling lower the air pressure to the recomended air pressure for a less stiffer ride. My old pickup I ran 80 psi in the rear when pulling my fiver and for everyday use I ran 50 psi with no ill effect on the tires.

Seems to me you're going at it backward. I'd take the rig, fully loaded and get at least an axle weight for each axle, preferably individual wheel weights. Then when I buy new tires, go into the tire manufacturers weight/pressure rating and pick the proper pressure for that axle loading. The Chevy recommended pressures are for the vehicle with driver and one or more passengers, running the OEM tires, not loading it as you are doing. Running a tire above max rated pressure shown on the sidewall is never a good idea, but OEM tires are not typically put on a vehicle to carry the loads you want to carry.