You should have two knobs that look like the knob for a PAC 1 directly under the dash. You can adjust them to the correct speed while moving. I used the speedometer app on my iPhone to calibrate it. This app uses gps to calculate speed. It was also dead on with my perfect pass.
Take a look under the dash on the left of the steering wheel and see if you see something you can turn. Then just remember where it is when moving across the water!
2006 Supra 20 - Sold
2006 Supra 24 Gravity Games - Sold
2015 Supra SE450 - Sold