Why not hook up to the tacho instead ? - Much simpler, and it's easy to find the tacho in most cars.

Please read the whole thread from the beginning. I clearly explain, in the very first post, that road noise is governed more by the road surface than by the car's speed or the revs of its engine. For me, at least, it's much more desirable to have road-noise-based compensation than speed- or tach-based compensation.