From a previous install with my AVIC-D3, tying into the speed sensor will make your speed reading more accurate than GPS alone. Pretty sure it compares what the vehicle's computer reads against what the GPS reads to make your final speed for traveling a little more accurate. Not required, just alittle more accurate when calculating arrival times I believe. Hope this helps!
Amplifier placement
- Thread starter BBQ BOY
- Start date
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what if you have 33's or 35's? Does it make it less accurate? 
The speed sensor feeds to your navigation unit for dead reckoning navigation. The modern Nav units use the comparison of GPS to Speed Sensor to calibrate the dead reckoning. Dead reckoning mode is used when satellite signals are low or blocked. It's important to make the connection for accuracy
