Steve I can tell you tires will last forever. We bought our dock at Lake Fork in the mid-1980's and about a year and half ago when Lake Fork got so low we were doing some metal support work on our dock getting it ready to sell and the tires were still there from the mid 1980's! We never put any tires in...they were there when we bought it.
We put this bad boy in when we built the pond. As the pond was filling with water I would have never thought it would catch enough air in it to float it, but it did. Had to pull it closer to shore where we wanted to be sunk and drill some holes in it with a cordless drill.
This monster was off a combine. Got into some fence wire that wrapped around the inner axle and rubbed the side wall off. dinked the tire for what it was intended for.
I second the caution about catfish spawning habitat. When I was a kid, a neighbor knew where there were a few tires in a local lake near shore (actually in front of their house). He'd catch catfish out of them when they were spawning. Now it's known as noodling. Their house is still there on the lake, although it's changed hands 3 times. The tires are still there.......
They are laying on their sides, up off of the bottom a little bit. There are a couple holes in the sidewalls. I have no idea what they used to put them there.
Some 5 or 6 years later, they are all still standing up as they were placed. The few times that the pond was fished, they were definitely magnets. I don't have CC (that I know of). - The holes at the tops were drilled with a 1/2" drill + bi-metal hole saw cutter blade. You don't want to drill thru the tread cuz the steel cords will shut you down toot-sweet. In fact, you have to be careful on the location of the sidewall; too high and you will catch the edge of the steel cords. You will know right away when you start the hole. I used a 3/8" standard bit to drill the holes for tying them together with UV resistant grade zip ties....the large/long heavy duty ties. Still holdin' today.
That pic is Sunil's Mound. His idea for the hump in the bottomscape.
Ewest prompted me to use the zip ties. I was worried for their longevity and he reassured me that he has used them for similar applications with good long term results. So far, so good. The other excellent tip was from Theo Gallus who told me to put a cluster near enough to shore to be able to cast to it (ere go Theo's Cluster) Major score on that one, too.
Any tire dealer would rather have you take 'em for nix than to have to pay a disposal fee. My source told me to keep my trailer load down to 18 or 19 max cuz 20 would technically require a hauling permit. (I can't remember the specific number)