I have installed the Micro-Trak Calc-n-acre on many pieces of farm equipment. For years I had one on a JD 4400. On the 4400 I placed the speed sensor on the rear steering axle and used the supplied magnet attached to the wheel. On some machines I have just duct tapped the magnet to one of the drive shafts leading to a drive wheel. This doesn't allow for slippage but still seems to work fine. On the 4400 I put a double pole switch in the sensor line and also put another sensor on the cleaning fan. By flipping the switch one way, I had ground speed and by flipping it the other I had a number indicating relative fan speed. This allowed me to reset my fan speed when switching crops. The drawback to this arrangement is that acres accmulated while I was setting the fan. For me this was not really an issue. I installed one of these for a friend on his 9500. He does custom work and was also interested in acres. On that combine we used a spring loaded brake switch on the remote RUN_HOlD line so that it only counts acres when the head is down. The unit is obviously not a Yield Monitor but for the price can be very useful.