Questions or bugs: hello@usedash.app
dash needs a GPS fix, which usually takes a few seconds outside or in a car and can take much longer indoors. If it never goes away, check that Location is set to While Using and Precise Location is on, in Settings › dash.
Most speedometers read a little high on purpose. Have a passenger open Settings › Match my car above 20 mph and type what the car shows. dash only changes the speed page; arrival times always use your real speed.
From how far you have left on the route and how fast you're going right now, so it moves when you speed up or slow down. When you're stopped it shows Apple's traffic estimate instead, dimmed.
Bugs, ideas, a place it can't route to: hello@usedash.app. The privacy policy and terms are here too.