I'm interested to see how this turns out with an older, worn concrete slab. I tried to do this on an exterior concrete pad a couple years ago with little success. The issue I ran into was the fact that the pad had weathered. The process of weathering leaches out the same minerals that the acid reacts with to produce color.
I'd imagine that the prep work is going to be a bear. That garage floor will need to be cleaned well enough in order to ensure a good chemical reaction. IIRC the acid stain manufacturers caution the use of muratic acid is it leaches out the necessary minerals as well. Process and pics to document along the way would be appreciated.