How about this: A newbie who can contribute something!
Here's the scoop on GPS stuff and tracking your speed. I do a lot of climbing and such and purchased a GPS a while ago. When I bought it, there was an intentional "wander factor" built into it, theoretically so terrorists couldn't use it to guide a missile made from household appliances into the White House. Subsequently, I believe it has been decoded and the system is fairly "accurate."
Even so, if you turn on a GPS you will note that simply standing still, you will have a velocity as it sits. Usually less than 5mph, on the order of ~2 usually.
That said, the GPS I purchased has the ability to record whatever track you have laid down over a given time, and though I haven't done it, I know there are USB cables and software that should allow you to calculate your speed.
This won't do the same as the data logger linked above (has accelerometers and such), but if you're looking to sort out speed over a distance, you can obtain that from a simple GPS. Additionally, mine has a display where ground speed is shown, and someone sitting in the car can simply watch that and take notes.
Last check, my GPS (Garmin ETrex) was ~US$100, the cable was ~$40US (I think), and the software was ~US$80 (I think).
Simple solution, fairly accurate.
Chris
Link:
http://www.rei.com/online/store/ProductDisplay?storeId=8000&catalogId=40000008000&productId=3576289&parent_category_rn=4500600
(no personal affiliation with REI, they just have the stuff)