I'd go with a 5Hz unit. If you do the math, 60 km/h == 60000m/h or 16.66m/s. At 5Hz,you'll be getting a fix every 3.33 meters which is pretty good. At 10Hz, it's every 1.66 meters. My assumption is roads don't change significantly under 3 meters (otherwise, your car would probably skid off the road).