I have a little 32cc rotary valve Berini clip on. It does maybe 400mpg and if it had a decent exhaust and an accurate ignition curve, it would probably do 50kph easily on the flat. It works best with a Shimano derailer machined onto a three speed hub, simply because you can help the motor at 50kph because you cannot pedal fast enough unless you have a very tall top gear. With a zener diode ignition it will go as slow as a brisk walk without any trouble. BTW the roller is 40mm garnet coated, but peak power is say after 4000rpm up. It is very torquey. The biggest problem is when fitted you lose your front brakes unless you fit a disc front brake. Motorists see that you are not pedalling and assume you are going twenty kph not 40kph. I also have a Ducati Cucciolo 50cc 2 speed running into a Sturmey Archer 3 speed. These things were actually raced in Moto Giro by tiny Italians on pure methanol and embarassed British 250s of the era. This engine is unique as the crank drives the cam which then drives the wet clutch and two speed gearbox. Worth a look at if you are curious of the origin of Ducati as a racer. The whole 4 stroke engine with two speeds and multiplate wet clutch sits between the pedals under the frame and uses the bicycle chain and any gears the bike has. The two bike motors first described are from 1948 when fuel and vehicles were in short supply, and in 60 years there is still not a better idea being sold for power assisted bicycle transport, just agricultuarally adapted single speed two strokes. Is anyone doing a purpose built computer chip managed electric drive bicycle running off a purpose built highly efficient low revving fuel efficient low slung 4 stroke stationary engine running an alternator/generator (hybrid using electric motor as a way of torque or revs variation instead of gears) with battery storage for solar panels or a wind generator when parked?