Are you on the Garage Journal forum, Paul? It’s a really good one for questions like this. Dozens of threads on the subject and a lot of helpful folk who have BTDT.
The standard for a concrete slab just for parking a Car on should be 4 to 6" and for vehicles of 2 to 3.5 Ton it should be 6 to 8" and for 3.5 to 5 Ton it should be around 8 to 10",We in the final design phases on our outbuilding and I'll be installing a two post vehicle lift. The manufacturer recommends slab thickness of 4 inches. We're pouring 5 inches for the entire floor. I'd love to hear from those that have done this and if you have any advice/suggestions? And any advice/suggestions on lift placement or "I wish I had done this......". Much appreciated!