Doesn't the system already check the latency from key fob to car antenna? Better yet, you could use two antennas in the car, and triangulate the key fob.
The latency check is usually not sensitive enough to prevent a car being stolen from the drive-way from a relayed key that lies near the front door.
For reference 20 meters is about 66 nano-seconds.
Why not? A 2GHz is very attainable in modern CPUs and translates to 0.0005ns per cycle. This isn't theoretical either. 802.11mc[1] is a real standard and is accurate to 1-2m.
Aren’t you off by three orders of magnitude? 2GHz translates to 0.5ns per cycle. Or perhaps you meant to write ms.
Whoops, you're right!
So, solvable?
UWB is augmenting Bluetooth for car keys solving this exact issue; The Car Connectivity Consortium came up with the Digital Key 3.0 standard that's available today, as implemented by some makes like BMW and the Hyundai/Kia group and Apple, and resists relay and replay attacks through precise ranging.