Originally Posted by prathmann

Yes, when you're riding you do slightly accelerate the bike with each downstroke and there's a slight deceleration in between. The extra weight of a heavy tire makes it a bit harder to do the acceleration. But the energy you put into it isn't lost since the spinning tire now acts as a flywheel and helps keep the speed up in between the downstrokes. So with light wheels/tires you'll get more speed variation during the course of a full pedal cycle while the flywheel effect of heavy wheels/tires will keep the speed more even. On level ground and assuming both air and rolling resistance are unchanged the average speed will be essentially unchanged. But, as mentioned, heavy tires tend to have more rolling resistance due to thicker tread and stiffer sidewalls that lose energy when the tire is compressed near the contact patch.