Tighter timing constraints could definitely work. If the car requires a response within 10ns you can guarantee the fob is within 1.5m of the car.[1] The only issue is the fob has to be able to decode the signal, calculate a response and transmit it very quickly (or within a very precisely known amount of time).

[1] Light travels at ~30 cm/ns, so a round trip time of 1 ns corresponds to 15cm.