The only downside I see to Bluetooth is some of the pairing lag. Might have to sit in your car for a few seconds before you can start the car. What about NFC? Something along the lines of this transceiver. http://www.ti.com/product/trf7970a?CMP=AFC-conv_SF_SEP
On this website that directs you to TI's website they offer a sample of the reader. Will need to make an account in order to get a sample. But something else to look into. Just use a token to start the car.
nfc transmitter and receiver datasheet and application note, data sheet, circuit, pdf, cross reference | Datasheet Archive
