I know guys who have done so, seems to work for them OK. I think your GPS will give accurate speed indications.
But.... (at least in my part of the world), to do so is not strictly legal. Which may or may not be relevant and/or concern you.
what if you lose Sat lock?
Then.... you'd have no indication of how fast you're travelling. My GPS only loses Sat in tunnels.
There are alternatives....
http://trailtech.net/vapor.html