Clutch engaged clutch. then loose 1/2 my rpm at full speed?throttle.

[FurryOnTheInside]

KCvale was telling us recently about one he uses that just goes against the spark plug thread. Fabian figured out a way to use a bicycle speedo mounted on the jackshaft sprocket.
I am sure with a single speed bike you can figure out RPM from road speed with a bit of maths or a gear calculator and then just remember what the rpms are on your bike at certain speeds, or write it on a chart and stick it to your handlebar stem.

 

[WINDY CITY BOB]

As long as we're on the subject of ratios and stuff like that, since I have a 49cc if I were to an 80cc? Would that give more power more speed or both or neither? Because I have another engine kit exactly the same as the one on my bike still in the box.
Thinking of maybe selling it and making a bike for speed only, a racer if you will.

 

[FurryOnTheInside]

From what I read here I understand it like this...

The 49 is supposed to be smoother than the 66, because it is basically the 49 bottom end that we get on both engines, with just minor changes AFAIK.
There is a saying "there's no replacement for displacement" certainly a larger engine should produce more torque and power, all things being equal (ATBE). Whether it holds true in real life (IRL) depends on the things that are not equal. Such as the upgrades you can do, spares availability both of which favour the 66, and the bottom end and associated vibration problem which favours the 49.
I would say 66 for torque, acceleration, pure grunt. 49 for smooth high speed on open roads.
I made my choice based mainly on the available upgrade parts and the wealth of knowledge and information on the forum, plus the fact that I want to run low RPM so I need the higher displacement.