Re: Gearing and Fuel Economy
Your goal is a bit far fetched at best... there's more than rpm that'll determine mileage and in some cases... especially in strikes with small engines, the way your going to get Max efficiency is by gearing it down not up.
If you're after the best fuel efficiency you need to also consider engine load, if you gear it up so high that it requires 3600 rpm to push it at 50 mph you're going to need a ton of torque to do so... a lot more torque than a 4hp engine can produce. You need to reduce engine load as much as possible thennrunnyour gearing so it'll cruise at the lowest possible rpm with the highest vacuum reading... the higher the vacuum the lower the load placed on the engine is. This is why motorcycles typically run at much higher rpms than a car. If you tried to gear a motorcycle to cruise at 3500rpm to do 70mph you would want that engine to be at or close to its torque peak so you would need roughly around 1500 cc's for the engine to run comfortably at 65 at that low of an rpm. That's basically about 80 ft/lb torque or so, but since it only requires about 20 ft/lb torque to maintain this speed, it can do it very efficiently and use less gas.
now lets say we have a smaller engine capable of cranking out 20 lb/ft torque at this rpm, it takes this much torque to maintain that speed at that rpm, but the engine is maxxed out doing so so the vacuum at this speed is zero and its gas mileage will suffer badly even tho its a much smaller engine and has enough torque tonmaintain that speed at that rpm.
Now what we need to do is multiply this torque thrum gearing so the engine only needs to produce lets say 7 lb/ft torque to move the bike at this speed but at a higher rpm like lets say 7000 rpm. The engine can maintainnthe speed but this time the throttle is barely open and the rpms are a lot higher and if a vacuum gauge was installed you would see about 1/2 to 3/4 the amount of vacuum you would read at this rpm but in neutral... now we're able to get better mileage because were not overloading the engine by gearing it too high for its output.
all my numbers in this are just figuratively speaking but if you get what I'm talking about you'll understand how to set up the engine and gearing so thebike can cruise at the desired speed with the least amount of load on the engine and at the lowest rpm possible. The easiest way to measure load is with a vacuum gauge. High vacuum = low load so you always want to gear for the lowest rpm that will give you the highest vacuum reading, this is the formula for getting the best fuel mileage.
We cant just gear something super high to get it to run a certain speed because there's always a point where the engine wont be able to reach or maintain that speed and the harder the engine has to work, the more fuel it needs to burn. Think of it like a 18 speed mountainbike... you can pit it in the highest gear at 10 mph but its not going to be that easy to pedal, but at 20 its not that hard... until you reach a slight hill or have a slight head wind, now you have to really pedal hard and its a lot of work, but you can down shift. Once you downshift you can.maintain that speed fairly easy, but if you shift down too far you're pedaling too quicknand working too much for the speed you're going.... your engine feels the same way so there's that happy meduim you need to find so it can maintain the speed you want but still have some power to spare.