The 66 cc has more power at bottom end, almost dont need to pedal to get going, but not good for the clutch. As far as speed the 66 cc seems to go a bit faster with more response at the throttle, still wasnt broken in when I sold it, had it up to 39 mph but pedal vibration was pretty intense. I weigh about 160 lbs. The Grubee just seemed to be a little better quality motor, with a better carb that has a banjo fitting, easier to use a thicker fuel line, unlike the straight up fittings that dont have any clearance. Best of luck. Chopper