CS may be pretty close on his 45PSI rating. I would keep an eye on the sidewall deflection when you are loaded, and if possible try to borrow a non contact temp guage. An underinflated tire will over flex the sidewall, cause an overheating condition and possibly delamination as a result. If you have a normal deflection and reasonable temperatures, the pressure may be apropros. If both are high, the pressure should be raised another 10 psi and the tires then re-evaluated.

You now have tires that can provide more load capacity than you need. It will take some experimentation to determine the correct pressure, providing the best compromise of protecting the tires, load capacity, and the vehicle's ride quality.

We know how horrible a blowout can be. We had tires that were only four months old with few miles on them, set with the proper PSI and load range at 14 ply/110 PSI...  they were Wanli tires, we bought them in Springfield, MO. at a well known tire shop... they are trash!!! We were near Robber's Cave in Oklahoma when it blew... I actually thought someone had shot a huge gun at us. We had to order (overnight) four new  Hancook 14 ply with heavy load range 3,280# per tire. $$$! Then we had to ship the bad tires to Joe's Tire in Springfield... where the sun didn't shine, to get credit for them. It wasn't a happy experience. Make certain you are covered. Our hobbies are too big of an investment to skimp on tires.

Not a real good plan when she has 'uprated' her tire considerably over the OEM spec.  Aside from the poor handling that will come from an 80 psi tire on a 4000lbs vehicle, it is almost certain the wheel is not rated for 80 psi and could fail (explode) if inflated that high.

You shouldn't have to worry about your stock rims or valve stems. Because it would not be realistic to run your new tires at their maximum rated pressure of 80 PSI, and instead, using a lower figure in the low to mid 40s, the stock OEM wheels will be sufficient for that purpose.

Your new tires have a considerable capacity advantage over the stock P radials they replaced. By not using all of their capabilities, you should not notice a great deal of change, in your day to day ownership of them.

Any "P" metric tire is rated as a passenger tire, regardlesss of the vehicle on to which it is mounted. It is commonly given a load index rating at 35 PSI and an equivalant of a four ply rating. The widths and aspect ratios will vary, and give correspondly different load ratings. The "P" series tires are often used on SUVs and Cross-overs to offer a comfortable ride on open roads. Generally, a majoriety of these vehicles are never be taken off the road, so the ride factor is generally favoured by the manufacturers and purchasers, over constant high loading factors.

Generally, a tire with a lower aspect ratio will offer better handling, because it lowers the amount of side wall flexing, and improves the vehicle's response especially in braking and cornering. However, the stiffness of the side wall will also determine how well a vehicle will respond. Typically the greater the sidewall strength, the more resistant the tire is to flex, even when aired down to lower pressures.

A LT tire has a far higher sidewall stiffness and load factor than a passenger tire. Lt tires in "E" range are given a load index rating at a 65 - 80 PSI. Even at a pressure in the mid 40's, you can expect to see the noticeable difference in handling you are now experiencing. The "P" metric was running near to the top of its load range. The LT tire is running only in the middle of its range.

Once you have established a tire pressure that is adequate and the tread is wearing evenly, you should enjoy both a high degree of security and a comfortable ride.