I think there are more stuff you need to take into consideration than just the thickness of the existing slab. The first thing I would be worried about is whether you have a tension slab foundation and garage floor. If you live somewhere that you have a basement in your house you probably don't, but if you live where you don't you may have a tension slab pad under your house.
In the photos below you can see the large red cables that span the foundation:
On my house new house in Texas they dug a ratwall about 3' feet deep around the perimeter of the foundation and also in a H pattern about every 8 feet that was 3' deep. Then they run tension cables across the foundation that is cable shielded in a plastic sheath. They also use a wire mesh which is not shown in these photos. When the slab is cured they tighten the cables up to pre-tension the slab to keep it from cracking. If you have a tension slab floor you have to make 100% sure you don't cut through them or you're screwed.
The other thing they did was place the plastic down as a water barrier to keep the water from making its way through the foundation since we have a high water table.
If you have a normal slab I would think of pouring a 24" x 24" pad that is tied into the existing foundation but you might want to think of digging as deep as possible (2 to 3 feet) and installing a 24" sonic tube over a bed of compacted sand/rock as a base material. From there tie your rebar on the top into a cage of rebar inside the sonic tube, essentially making a small pillar underneath the new section of floor you are pouring.
Might be way overkill but I believe that sometimes you can never over-engineer something, especially the footing for a hoist to hold a $50K+ car
Or you could just buy a 4-post hoist...............