Usually my tach reads about 3000 at 60 mph in 4th gear and 3500 at 70. When I turn on the heater/defroster fan the tach reads higher. The faster the fan is running the higher the tach reads. If the fan is on high the tach reads 600 rpm higher than normal. With the fan on low the reading is 200 to 300 high. All other electrical things work fine. Engine grounds are good. Any ideas???