A device and corresponding method for enabling a user to accurately control, monitor, and evaluate performance of a vehicle. The system includes a portable programmable computer device that a user can readily install by plugging it into a diagnostic connector port of the vehicle for providing the controlling, monitoring, and evaluating functions. The present invention enables a user to perform performance tuning of the engine, provides dynamometer type functions such as horsepower and torque measurements, and provides a virtual drag strip function for accurately measuring acceleration data. According to an alternative embodiment, a monitor mode is provided for enabling the user to monitor selected characteristics of the vehicle and a diagnostics mode is provided for providing for the identification and display of diagnostic trouble codes (DTCs) with informative text descriptions. The present invention also enables the clearing of selected DTCs without requiring expensive customized equipment for that purpose.