Google "My Tracks" .... it's free. Once you start recording your track, it will overlay on a sort-of Google Map looking map. Click on the screen, then click one page to the right. It's all the stats about how fast you're going, how far you've gone, etc.
I just drove cross country with my Droid in a moving truck that had a broken speedometer. I started Google Maps, then started Navigation (so I could hear the turn by turn navigation prompts). Then I opened the Google My Tracks app and started recording my "Track". After it started recording, I went to the second page and instantly had a working speedometer!
+1 for Google and Droid!
Best regards,
Brad