if for some reason the installer tapped into the OBD-II VSS signal? seems strange they would do that but maybe it was the only option... I've never done an aftermarket cruise install but if you parallel to applications off the same signal they will drag each other down unless you have a way to split the signal and keep the voltage up.
2007 Honda Civic Ex
Second Goal = 50mpg
First goal = 40mpg Goal Achieved 3 tank average over 40mpg
Starting point 30mpg ready...... GO.