Ris91, the tachometer displays the engine speed (aka RPM), not the wheel speed. The horsepower and torque ratings for the vehicle are for the engine at a particular engine speed, not ratings at the wheels. For those numbers, you'd need to have your car tested on a dynamometer (aka dyno). The vehicle speedometer measures the wheel speed in miles per hour or kilometers per hour. This is not a direct correlation to the tachometer. The vehicle speed is usually measured at the output of the transmission or via ABS wheel speed sensors. Because it is not measured by the outside circumference of the wheel, the speedometer reading can vary if non-stock tire sizes are used as well as by the air pressure in the tires.
-Rod