I thought so far that every NC2 has "trip computer" showing mpg and ambient temperature. But no, it was an option. But it can be easily enabled using Forscan (registered)

Jeroen / MX5Info showing how to do it

MPG display works immediately. To have ambiend temperature working it is necessary your NC has sensor. It is just behing front bumper, on the left side. Cars having automatic AC have it installed from factory, others have only wire harness and plug. Can be easily retrofitted

it works on NC2 only, earlier NC1 cluster does not have "trip computer" functionality