The speedometers in cars is sometimes are inaccurate, due to them counting speed as based on it calculating from how fast the tires are spinning.
That also takes into estimate the size of the tires... Load slightly bigger or slightly smaller than your original tires, and the speed displayed becomes a little bit more "off."
If your car is sliding, as was the case a month or so ago after it snowed and there was a big patch of snow in the mall parking lot that my friends and I had some fun with (Car-sledding), then the speedometer will be WAY off.
Anyway, having speedometer software on the PocketPC wouldn't be a bad idea. If your car said you were going 50 but you were really going 60 in a 55 zone, a GPS would tell you, but your car's speedometer might not. It might save ya from a traffic ticket….
Unfortunately, no, I don't know one that will display it in large text.