I know mechanical speedos can fluctuate due to a few different reasons. I have never seen a modern electronic speedo/tach fluctuate personally and to me would indicated something electrical. Never seen my colorado gauges do that before.
2015 Colorado Red Rock Metallic CC Long Bed Z71 4x4
-Mods So Far: Rally Armor UR Plus Flaps, Yellow Tinted Fog Lights, Access Vanish Cover, Linex, EGR Matte Black Wind Deflectors, P3 Brake Controller, MBRP Stainless Steel Cat-Back, Anzo LED Tail-lights, Uniden Bearcat 880 CB w/3' Firestik, Falken ATw3 Wildpeak 265/65R17 tires
Self Diagnosed RC Car/Truck Addict