I use the ridewithgps app on my phone. The speed readout varied wildly so I increased the interval for averaging, which made the updates slow. I do read it and find it useful as long as I remember that it's not going to be very accurate and to the degree that it is accurate, it tells me my speed a few seconds ago, not my current speed. I think there is some recalculation after the ride's end, because overall average speed changes after I stop the ride.

I've been thinking of getting a speed sensor, but I'm not terribly motivated to go through the expense and trouble. It's not that important to me.