My 4 stroke friction drive will go 18-22mph all day, and that's good enough for my mountain bike. As you said, it is an assist for when I don't feel like pedaling. A rear mount/friction drive can also be hidden behind some false touring bags to operate in stealth mode. Those little gas tanks they come with won't get you very far either. I carry a couple of the 30 oz MSR fuel bottles for extra gas, but I want to set up something more permanent that runs direct to the engine. I do need to get my motorcycles going again though. I miss riding them. Too much of "life" has been coming up lately. My CT90 frame looks at me every time I go into the workshop, just begging for the 140 Lifan engine to go in. I guess I need to just get busy!