I guess you will use wheel speed from an inductive sensor and vehicle speed from a GPS antenna, right?
Go to "tools" => "Pi Math" => "edit..."
In the window, click on the "gree plus" and you have created a new channel, name it and in the window you will type the formula, eg:
Then check the units, date rates and you are ready.
NOTE: if you use the "speeds" I mentioned, the calculation may be wrong because of the way wheel speed is calculated (everchanging diameter of the tyre is involved). Anyway, it worths a try