Wouldn't it be very exact if you somehow measured a short period of time and the distance traveled in that time?
Like use 10 seconds and if 10 Km was passed in those 10 seconds you could report that 60 kmph is the speed. I guess it wouldn't really be a speedometer as much as a cruising speedometer... but
I have no idea about how the gps works, but if it reads within 1 meter of accuracy you could even go down to tenths of a second and just do the math to convert it "per hour".
Just some random thoughts, sorry if it's not helpful.