There is no need to use two separate speed senders. You may share one speed sender (sensor) between the cruise and the speedometer.
Or better yet, if your after market cruise control will accept or read a 5v square wave signal, you may actually use the “OUT” terminal from the speedometer to supply the cruise control. In other words, run your signal from the speed sensor to the “SIG” terminal of the speedometer and then run the signal for the cruise control from the “OUT” terminal of the speedometer.
Now that you have signal to your cruise control, you may need to know how many pulses per mile (PPM) your vehicle, or speed sensor puts out. Since you are using the AutoMeter Electric Speedometer, there no longer any need to perform any math. You don’t even need to know anything about the number of pulses the speed sensor produces per revolution. Every time the speedometer is powered on, it will momentarily display a number before defaulting to the odometer mileage. This number is the calibrated PPM. After you perform your speedometer 2 mile calibration as outlined in the speedometer instructions, it will now display an accurate PPM upon every power up.
From the factory, AutoMeter speedometers are calibrated to 16,000 PPM, so when the speedometer is powered (prior to calibration), you will notice 16000, then whatever the current odometer mileage is.