One hour active-engagement tutorials using microcomputer-based laboratory (MBL) equipment were substituted for traditional problem-solving recitations in introductory calculus-based mechanics classes for engineering students at the University of Maryland. The results of two specific tutorials, one on the concept of instantaneous velocity and one on Newton's third law were probed by using standard multiple-choice questions and a free-response final exam question. A comparison of the results of 11 lecture classes taught by six different teachers with and without tutorials shows that the MBL tutorials resulted in a significant improvement compared to the traditional recitations when measured by carefully designed multiple-choice problems. The free-response question showed that, although the tutorial students did somewhat better in recognizing and applying the concepts, there is still room for improvement.