GPS speed is typically specified at 0.05 m/s accuracy which is the equivalent of +/- 0.11 mph. Any GPS you find will most likley be close to this in terms of speed accuracy.
As for a 5Hz GPS you can get similar or even better performance also with a standard 1Hz GPS in combination with an Arduino. If you keep track of acceleration/deceleration (difference in speed between readings divided by time) you can account for this and refresh your speedo at 10Hz or more if you think it makes sense. There is significant inertia in a vehicle so at moderate to high speeds, such predictions will be highly accurate.
Unless you find a 5Hz GPS that is the better choice in terms of price, size or power usage it does not really add value for this type of application.