Understanding gear ratio and tire profile is essential to calculate exact speed of the bike at specific gear position.
Modern motorcycles already have too much horsepower on it. The simple and effective way to increase fuel mileage is to put taller tire, maintaining the max tire pressure. To have serious fuel saving, changing gear is good enough without any engine or fuel system modification.
The overall gearing of a motorcycle consists of 3 major parts:
All these ratio combined results in the Total Gearing Ratio of the drive train:
Changing any of these parts or changing the rear wheel tire‘s dimensions will effect speed and / or torque results. All of these changes can be simulated by using the GC by changing involved gearing fields. To be able to do this correct, all ratio, sizes and RPM’s are necessary input.
The primary drive is the ratio between the engine’s crankshaft and the incoming shaft of the gear box or clutch. Usually this ratio can not be easily altered unless you perform major surgery on your bike…. But if you do, also change the Primary Drive Ratio field in the GC accordingly.
The gear box is the only gearing part that can change its ratio very easily and that is by you changing the gear while you drive. Normally a gear box is dedicated for an engine or bike so you do not have to change gear ratio in the gearbox itself. But sometimes the stock gears are not suited for all circumstances and manufacturers offer a way to change the internal gear box ratio by either offering different cogwheels or by offering a so called cassette gearbox that can be swapped easily. That is why in the GC you can change the gear ratio of each gear by changing the white gear box ratio fields for each gear.
The Final Drive is sometimes also referred to as ‘the product of all ratios involved’ but I prefer to call that the ‘Total’ or ‘Overall’ ratio. In our example the Final Drive ratio is the ratio of the final part of the gearing, the ratio between Rear and Front sprocket. This is also referred to as ‘Secondary Ratio’.
Below a table containing the possible final drive gearing changes and their results on Speed and Torque.
As a rule of thumb, changing the front sprocket with 1 tooth, amounts to the same effect as changing the rear sprocket with 3 teeth. This does not exactly apply to every bike but as a rule of thumb it will do and explains the results in the table below.
As you can see, the effect of adding a front tooth and keeping the rear the same has about 3 times more influence on the speed then removing 1 tooth in the rear and keeping front the same. Changing only the rear with 1 tooth does not have a lot of effect, in combination with also changing the the front it either amplifies or weakens the total effect.
The exact increase or decrease for your bike can be calculated using the Gearing Commander tables.
Combining all these individual ratio like they are in a motorcycle, results in the total drive train ratio or the overall ratio. Just multiply all the ratio to get the Total Ratio: Primary ratio x Gear Box ratio x Final Drive ratio
As the rear tire is a major part of the total bike gearing, changing its size does effect this gearing and thus effect speed and torque at certain RPM’s. This is easy to explain: the chain (or drive shaft) is rotating the rear wheel at a certain RPM. Now when it is a small wheel in diameter it will have a small circumference and when rotated it will cover only a small distance. A bigger wheel will cover a bigger distance when rotated equally fast.
So changing for instance your rim size from 16″ to 17″ would mean a higher (top-)speed at the same RPM.
Now the height of the tire is a percentage of the width. A modern tire has these size marks on it 190/50/17 which means: the width is 190 mm, the rim size is 17″ and the height of the tire is 50% of 190 mm (=95 mm). This tire has a circumference of: 1953.4 mm
More info on Tire Circumference