The latest beta patch (1.15) addresses this:
QUOTE(Suma)
There is one feature I would like to highlight, and that is incremental mouse bindings, which allow more natural vehicle steering using mouse (no autocentering when steering).
To use them you need to bind them manually:
Unbind Mouse left / Mouse right from your Car left/Car right
Bind Mouse left / Mouse rigth to Car more left / Car more right
Incremental mouse bindings currently work for cars only, other vehicles (tanks, boats, bikes) are to be added in the final patch.