I have an app on my iPhone that does that. I used it while I was waiting for my new speedo gears after I changed the rears from 3.0 to 3.8.
You place your phone on top of your dash board facing up and you're good to go.
Worked pretty well. One thing I didn't like was that for it to work you need to give the app access to gps tracking so it can see how fast you're going and I don't like apps tracking me.
But it was both easy to read and accurate.
However the phone's screen would sometimes make a reflection in the windshield which bugged me a little.