[f5fstop]

No gear to switch out as on Wranglers (excluding Rubicons). The H3, as with all GM vehicles, uses an electronic vehicle speed sensor, located in the tail of the transmission. Is is a magnetic sensor that detects openings in the teeth of a reluctor ring.
This signal is sent to the PCM, which uses this information for control of the engine and transmission. It also sends this signal to the BCM, which calculates vehicle mileage and sends a class 2 message to the I/P. The I/P will display the speed and miles.
Run with a GPS and you will be able to determine how much the speedo is while driving.
It is progressive, the higher the speed the farther off the speedo will be.
My jeep, with tires two sizes larger is off 4 mph at 70 mph per my GPS units.