Cutting up a steel-belted car tire is a difficult proposition. In the first place, those steel belts are more like steel-wire meshes. The mesh alternately catches and flexes, making it difficult for the blade to cut. Industrial-grade cutting machines can take care of the job in a second but cost far more than the average hobbyist can afford. If you're making a rubber tire planter or bumpers for your boat dock, here are some suggestions for how to handle those steel-belted radials with your own tools.
Mark where you want to cut the tire. The difficulty of managing the blade as you cut will make it hard to stay on track if you don't mark where you want to go.
Drill a starting hole through which to insert the reciprocating saw blade. Don't try cutting into the sidewall through the bead -- the inner rim of the tire. The bead is a thick bundle of wires that will wreck most saw blades in a second. To make cutting the desired pattern easier, drill a starter hole inside the bead and cut around the inside of the tire along the bead, removing it like a doughnut. With the bead gone, it will be easier to cut the rest of the tire.
Insert the reciprocating saw blade into the hole and begin pushing it along the line you've marked. Push slowly to prevent the blade from binding.
Drive a wooden wedge or chisel in behind the cut as the blade moves forward to force the rubber apart and prevent blade binding. Turn the saw off while resetting the wedge.
Remove the waste part of the tire from the cut in sections to keep the gap from closing behind the blade and binding. This is especially important when cutting tight turns or bends. Don't force the blade to move too fast when changing directions or it will bind. Repeated binding will only slow you down.
Work slow. Use the right blade - smaller teeth tend to work better.
Wear eye protection. Wear leather gloves to protect your hands from sharp wire tips that will become exposed where you cut through the steel reinforcement. Always turn off the saw when freeing a bound blade.