Co-ax V 300 ohm flat twin.

[Terry]

We used to use twisted pairs at work and that generally worked out at about 120 ohms.

As the impedance is related to the log of the ratio of conductor diameter and spacing multiplied by a figure derived from the relative permittivity of the insulating material, varying any one of the three parameters can have quite an effect on the result.

Assuming you are referring to telephone type twisted pairs, the conductor size and spacing will be considerably smaller than for twin flex, although the ratio between the two might be very similar - who knows? Even then, variations in the relative permittivity of different insulating materials can affect the result.