We used tires to sink a stack of five pallets for a scout project with AZ GFD. This is their suggestion for weight.
1. lay the tire flat on the ground
2. cut a circle of plywood to fit inside the tire without falling through
3. cut the circle in half and insert both in tire
4. now you have a mold to fill with concrete
5. stick a U shaped piece of rebar in the concrete to fasten to whatever you decide to sink
A nice feature in this idea is once the concrete has set, you simply tip it on its side and roll it
to the truck or trailer. No carrying