12 Cruze LT with 6spd, 56K miles. Not sure if this is the right place to post this, but here goes. This has happened a half dozen times or so now. While the car is idling in neutral, not moving, foot on the break, both the digital and analog speedometers will jump, thus increasing the radio volume (as I have the speed compensated volume set at medium). It's happened mainly to my son, but finally did it to me last week while I was stopped at a red light. I noticed it as the music volume got louder, which caused me to look down at the radio, then the gauges. The speedometer went up to 60mpg, then dropped down to 56 and held there. I honestly cannot remember if the tach jumped up as well, as I was shocked as to what was happening. I flipped through to the trip odometer, to see if it was adding mileage; it was not. When the traffic light turned green I put it in gear, and the speedometer dropped right down to zero, along with the music volume. Any thoughts as to what it might be? I'm not taking it into any dealer for them to charge me to look at it; there is no other warranty except the factory powertrain warranty. Thanks in advance.