= When enough excess power is available the rig is powered on and you mine for free.... In your car!
LAWS OF THERMODYNAMICS = there is no free power!
You will be burning more gas/petrol to power it.
Unless that power is already being generated by your alternator and wasted. Then mining with the excess makes you more efficient. <.<
I like the idea of an automotive-based hotspot network utilized by mining hardware.
An alternator produces UP TO ~100A.. meaning it doesn't continuously put out 100 amps when the alternator is spinning. If it did, assuming it's 75% efficient, you'd be drawing 100A/0.75 * 14.4V = 1920W @ 746W/HP = 2.57 horsepower your engine is putting out that isn't going to motion of the car.
Cars are about 35% efficient - 30% in heat removed by internal cooling and the rest out the tail pipe (why turbos are wonderful)
1 gallon of gas contains 131,760,000 Joules - @ 35% efficiency, you get 46,116,000 Joules of energy out of.
46MJ ~ 12.81 kWh.
@ $4/gallon, 1 kWh costs $0.31/kWh
Also don't forget to apply alternator efficiency on top of that too. => $0.31/0.75 = $0.41/kWh
YMMV