[dcb]

If your gti has cruise control, it probably has a vss already, finding it in the schematic and tapping it in the car is the biggest challenge.

if no vss then you need to install something if you want the guino to track distance. The simplest conceptually might be to take a bicycle computer and strap the magnet to an axle shaft (the inboard most end would be best) and make a bracket to hold the switch near it, then wire it up to switch 12v and feed it to the guino.

the reed switches are a little noisy however, so you would be better served with a hall effect switch, but I don't have a circuit diagram handy.

opto might be another option.