the bike computer speedometers are slow to refresh but just as accurate as you program them to be. You just gotta measure your diameter of the wheel and tire as accurately as possible, and then plug in the diameter, circumfrence, whatever.
i think they refresh like every 1.5 seconds, which aint bad for a speedo... if i had a tach i'd want it to be analog or refresh _much_ faster than that, but for just mph, youre probably not fluctuating that much in the first place.