I would just start with something simple and play around with it. That something simple could be something like interpreting the input as an angular velocity applied to the steering wheel. Cap the position of the steering wheel to make sure it stays within [-MaxAngle,+MaxAngle], and add a restitution of some sort when the keys are released.
Test this scheme at a fixed, typical speed, then see if it needs any dependence on the speed.
Well, some actual car manufacturers implements sort of steering wheel sens. reduction with speed. That is done for not allowing car to flip or went out of control on speed turns.
I'm happy how car behaves in turns with no aids. But it's getting pretty messy when it trying to, say, change the lane on highspeed. The project I'm working on have pacejka implementation for wheels. I've talked to some experts, they suggest to find wheel's perfectgrip angle (where grip at it's top) and since then, if the speed more then some variable, apply a small amount of steer to wheels.