A system for monitoring and controlling operating parameters of a moving vehicle comprises a device adapted to be removably attached to a vehicle diagnostic data port. The device communicates through the vehicle diagnostic data port with various vehicle electronic control modules (such as an engine, transmission, exhaust, or other control modules). The device monitors vehicle operating parameters or conditions to detect when vehicle performance or efficiency or operation may be altered in a useful way. If so, the device commands the vehicle electronic control modules to alter the normal vehicle behavior to perform a different behavior. The device may be removable, and may command a moving or stationary vehicle. The device may also monitor and control an aftermarket exhaust valve attached to a vehicle, based on monitored vehicle operating parameters and conditions.