Maybe I am missing something here but once the GPS signal is received, isn't speed automatically calculated as a function of the GPS? At least on my unit it is.

However, I have something called a HKS type-1 Turbo Timer on my car (http://www.hksusa.com/products/more.asp?id=845). Among other things, it calculates performance measuring metrics like 0-60 mph, 1/8 and 1/4 mile times (sorry, big performance metrics in the U.S.), etc. When I installed it, one of the connections was to the speedometer lead on the ECU so obvioiusly the signal is available and this unit can go on any car. I have not idea what the pulse looks like to it seems to be the same for most if not all cars. I'll try and put a scope on the signal and see if I can get a digital picture to post.