update. went out and rode on a walking path with rough measured distances. I went 1.5mi according to path and 1.9 according to my computer. the path is wide and arced, but no corners. I then put in 180cm, rode the path .9mi and my computer said .87mi. i put in 183 when I got home(started raining) so I will try again another day, but I am close, but off the recommended chart. i will also try somewhere with more accurately measured distances.