I disengaged my nano from the car and just tinkering around with it. I'm pretty sure I may have fried it. When I connect it to ground and a 5v signal on an IO pin the onboard LED turns on without connecting to a Vin. Maybe the tachometer was a little higher than 5v and burnt out the IO pin. Don't think it was anything wrong with the nano, just my circuit design. Thanks for the relies, I'll direct my further design questions on a different thread.