Smartcraft SC100 Speedometer hooked to SC1000 Tach

[nates90]

I heard that the SC100 gauges piggyback the SC1000 gauges. My question is if I get a kit that has the SC100 fuel, trim, speed, then I have a SC1000 Tach that reads those gauges, how does the speedometer reed the speed from the motor information. I really don't want another GPS antenna on my boat and I get speed from my Humminbird anyways, but would be nice to have a gauge on the boat. Thanks.