If you do not limited in budget you may look towards Garmin marine or aviation unit because its have Built-in high-sensitivity 10 Hz Garmin GPS/GLONASS receiver and you can see changing of speed more often and precisely. Also the external antenna is desired for good precision and stability. On all these device there is window (page) with very big digit for speed on screen only.
Garmin GLO is more cheaper but it is required an addition iPad, iPhone or Android device with navigation program.
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