The cruise speeds on 150DM are probably of no interest to you, but standard 150s I've flown along with seem to average around 115 mph generally speaking. Airspeed indicators are frequently off by a good 10 mph, and more often than not read high. One practical way to know how accurate your ASI is would be to use a GPS on as calm a day as possible and fly at given speeds in various directions, noting any differences between the GPS and ASI. If it's a calm day the only material differences should be the error in the ASI at low altitude. My ASI has always been as close to accurate as any I've seen, as is the tach. It's good to know what these two gauges are really telling you for your individual airplane.
