Thank you Coyote X for the encouragement to try the capacitor, and dcb for the VSS Delay idea. I will try upping the VSS Delay on the mpguino if I need to. The capacitor seems to work!
A Successful Debouncing of 1996 Geo Metro 1.0L w/ 140,000 miles:- 100nF (same as .1 μF in radioshack units) ceramic disc capacitor connected from VSS to ground.
- 56k resistor connected from switched 12V to VSS
Now the speed is stable at 40+ MPH. Before, using only resistors, I was still getting bouncing of the speed in 40+ MPH range.