[Wildkarde]

For the s10 mod to work your pcm is going to have to be correctly calibrated for tire size/gear ratio. The speedo output on the pcm outputs a 4000 pulse per mile signal, but it is based off of the miles per rev calibration in the tune.

If you are going to go aftermarket you dont need gps though, any of the modern aftermarket stuff can be set based on a pretty much arbitrary ppm signal.