To calculate the gear ratio divide the number of teeth on the main gear by the number of teeth on the pinion gear.
Examples: Trex700 main gear 115T Mode 1 / 17T pinion= 6.77 gear ratio
JS Frenzy TZ90 main gear 91T Mode 1 / 14T pinion= 6.5 gear ratio.
Gear ratio is a factor to obtain desired head speed in relation to optimum engine power. As a guide a scope between 6.0:1 & 7.5:1 is used for gassers using the two stroke Zenoah engines. In brief too high a gear ratio equates to the engine working too hard at lower head speed which in turn increases fuel burn & vibration which will lead to excessive wear & tear not only to the engine but servos, bearings & gyro performance. With a lower gear ratio you target higher head speed at lower RPM resulting in better fuel efficiency but if the ratio is to low you can experience loss of power during certain manoeuvres such as acrobatics & 3D which is related to the engine operating too far from its peak power RPM even though it may be set to run at max torque rpm range.
Head speed is measured with a tachometer at the blades in RPM. Head speed is basically the RPM measured at rotor blades after gear reduction (main gear & pinion gears). Head speed should be aimed to run within the power band of your particular engine, you will need to referee to test data sheets, performance curves or contact machine shop if worked.
Head speed * gear ratio = Engine RPM
Example 1810 RPM (head speed) * 6.77 gear ratio = 12254 RPM
Engine speed (RPM) divided by the gear ratio = Head speed RPM
Example: 12254 RPM / 6.77 gear ratio = 1810 RPM head speed
The Zenoah manual states maximum efficiency (power band) for the stock Zenoah G230RC engine is between 10K RPM (max torque) and 14K (max power). You want to stay below 13000 RPM with the stock Zenoah engine due to vibration issues, & any slight power gain over 13K, will be offset by wear & tear, fuel consumption, & excessive engine vibration which in turn will affect aspects such as gyro performance etc.
The following Head speed guide values are based on stock Zenoah engine with maximum engine efficiency between 10K and 12.5K. They are only an intial guide value with a degree of safety margin.
In order of gear ratios: 6.5 - 6.77 - 7.0:
Like you are now accustomed to adjusting throttle & pitch curves, the same goes with head speed. The values are an initial guide only & need be adjusted to specific requirements such AP work, scale flight, weight, gear ratios & engine type i.e. modified.
Most scale & AP flying is done at lower head speeds than for 3D or even sports aerobatic flying.
For smooth scale flying, less cyclic pitch helps to also smooth out response.