A device and corresponding method for enabling a user to accurately control, monitor, and evaluate performance of a vehicle. A portable programmable computer device that a user can readily plug into a diagnostic connector port of the vehicle for providing the controlling, monitoring, and evaluating functions. Accurate detection of the start time of first movement of the vehicle is made based on data from an accelerometer, independent of the onboard diagnostic system and its diagnostics port. A clock is started for measuring time such that the precise time taken to reach the time-stamped first velocity value from the engine computer via the diagnostic port is determined. Data from the accelerometer and diagnostic port is analyzed and selectively used for accurately and reliably correcting for errors in velocity data from the onboard diagnostic system including latency error and errors due to the vehicle exhibiting wheel spin.