No I'm saying that that's not how you get the wheel spin "number" as you'd need to know how fast the bike is actually going. The sensor on the rear wheel OR engine RPM only tells you how fast it SHOULD be going, hence the need for the actual bike speed. Either via the front tire as it has no power, not the most accurate way though as it's not always on the ground (wheelie) or it's sliding.
My source is personal experience, no seriously it is.
![Tongue :P :P](https://www.dirtbikeaddicts.com/static/images/smilies/tongue.gif)
Heavy farm equipment (tractors) will a lot of time have a wheel slippage indicator, which gives you a good reference point on how deep to set whatever you're pulling (disc, plow, etc.). The way the one I always used worked, is with a sensor on the rear wheel to get the true wheel speed and a radar gun mounted underneath getting the true ground speed. It would always make the police radar detectors go off.