I know on my little 2.0L 4 banger, the drop in gas mileage would be more expensive then just paying for the electricity.
Really. The energy has to come from somewhere, and a small ICE along with a cheap alternator are absolutely crappy at turning fuel into electricity. This graphic is actually pretty good since it's 40% conversion of chemical energy to mechanical energy is for a big diesel; for a small gas ICE it is actually very decent.
Now, you could get almost free power if you tied your mining rig into your ECU. While accelerating you wouldn't actively hash but would just keep valid work queued up. As soon as you tap the brake, start hashing. The increased draw on the alternator would act as a form of really expensive engine braking and would save wear on your brake pads.