To answer the OP's original question: Yes, you can run an ad hoc network off of any connection. Your phone becomes your "ISP", your laptop becomes your router, and you can then network to anything in your car.
Downsides to this:
1. It is against every TOS from EVERY cell provider to do this
2. Your downstream usage would explode and eventually your provider (at and t) would drop you for overuse.
3. Your hotspot would be more of a luke warm one. Edge sucks for downloading, chopping that download up between two or three devices connected to your "network" won't make it any better..