Before: 1980 Puch Maxi. Stock bottom, aftermarket piston, hi-torque cylinder/head, techno estoril, 16/45 gearing and 14 bing, was hitting 31 mph.
After: roller bearing crank, 3 shoe clutch (getting ready for a kit) and the only other thing I changed was going down to a 14/45 gearing. I am now hitting 36 mph.
Problem was, with that taller front sprocket I was not letting the engine hit a higher RPM so the pipe could do its job. Now it has great acceleration AND good speed.