I think it has an excellent representation of speed. I fly Cessna 2,3 and 4 series and have about 2,800 hours. Try this, check out the DME and fly for 4 miles at 240kts, which equates to 4 miles a minute and see what you come up with. I assume the demo has zero winds and make sure you're using Groundspeed. Indicated or True airspeed will be different depending on altitude. Use groundspeed and try different altitudes, you should come up with the same times, assuming no winds.
Curtis
Montana