LAWS OF THERMODYNAMICS = there is no free power!
You will be burning more gas/petrol to power it.
I like the idea of an automotive-based hotspot network utilized by mining hardware.
except any real excess power you generate isn't going to be able to power a mining rig + by mining on the rig in the first place you increase the load on the alternator which increases its resistance, which makes the engine have to work harder, which makes you burn more gas.
It isn't going to be free, but when it comes down to it, if its some USB miners and an RPI you probably aren't going to notice the little bit of extra gas they'll run you.
What were you planning on doing for network? Just wifi + hotspot on your phone?