Update on my tests with the Dakota Digital device...It kind of worked, but not really.
Short story: I installed the device on the vehicle speed sensor wire going to the ECU. I set the ratio up as high as it would go, which was output signal was 4x the input. Started driving slowly down my street, and unfortunately the speedometer was unchanged. However, the ECU definitely changed, because going ~10 mph in 1st gear the instrument cluster was telling me to shift into 5th or 6th. Also, the power steering was much tighter making it more difficult to turn the wheel, because the car thought I was going at highway speeds.
Long Story: I had purchased a 1 month subscription to the Hyundai Tech Info site so I could look at the wiring diagrams to install this.
Based on the schematic diagram, the speed data starts at the 4 wheel sensors and goes directly to the Electronic Stability Control (ESC) which I guess is Hyundai's term for anti-lock brakes.
From the ESC, the speed signal of the front right wheel is output to the ECU and IBU. This is the signal that I tapped into.
However, the speed data that goes to the instrument panel doesn't come from this output. It comes from the ESC over the vehicle's CAN bus.
This is confirmed in the write-up of the system.
Now I'm no expert on these things, but from what I understand CAN bus is not just a simple signal that can be intercepted and changed. It's actually a data network with packets that flow to and from all the systems in your car. So unfortunately, not something that can be easily changed. 😢