i am assuming construction, and having problems with nails and sharp edge objects puncturing out tires and reason for high ply number.
as far as tires go. get the old paper phone book out.
and start calling "tire" places. you will get a few automotive places that don't sell or deal with tractor tires. but after a few calls to the local tire places you will find someone that knows "who" to call. that deals with local farm tires.
you need to call a few places, different places only sell in certain manufacture tractor tires. some are very very costly, while others are half the price as some other manufacturers.
pending on how you want to do it. take the tires off tractor and take them in. or pay a small service fee for the company to send someone out and do the swap out for you.
after i hit the phone book, and seeing price difference online. i ended up going to a couple different yellow pages websites. to search in about 150 mile radius of me. and ended up finding someone that was 300 bucks cheaper than everyone else. that included milage, service fee for both tires for mounting/unmounting, tubes, plus new tires. and kicker was, there was only a service fee and no hourly charge fee. so it made my day. and the truck already had a tank in back, if i wanted to get my tires filled with some sort of fluid for weight.
this was for 28x18 ply10 R4 tires. on the 555c loader/backhoe.
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they guy that took off the tires for the loader/backhoe actually preferred the tires stay on the tractor. so he could use extra force. to pull off old tires and put new ones on. and just used his jack to lift the tractor up a little bit.
after watching the guy use pry bars, that didn't have any sort of point of them, and then the muscle needed to pry the tires off the rims and then get them on. i would most likely leave it to the tire guy to switch out tires. if i tried it, it would of been more like a entire day project if not 2 days. vs pro that was like 2 to 3 hours.
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when looking for tires for myself i found R4's range from ply 6, 8, 10, 12.
when looking at online websites to buy tires. i found a couple companies that bought 2nd hand tires. or rather tires that were put on by manufacturer, but when person bought tractor they wanted a different type of tire. so basically new tires. but time you counted in shipping, and sight un-seen, and the price. pay a 100 or so bucks more, for local doing was worth it to me.