To squeeze out the limit tail speed of a motorcycle or chain-driven vehicle, the most direct and effective means is to adjust the gear plate: either increase the front gear plate or reduce the rear gear plate. From the logic of powertrain modification, this operation is to “lengthen” the transmission ratio (for example, from 3.00 to 2.75), so that each revolution of the engine can be converted into more wheel rotations.

The effect of adding tooth to the front toothed plate is roughly equivalent to reducing 2.5 to 3 teeth to the rear toothed plate. The so-called “best” size is by no means a brainless change of the largest plate. The ideal gear ratio is the balance point where the horsepower curve of the engine can just pass the air resistance. If you adjust the gear ratio too “Tall”, the engine will not be able to carry the highest gear due to insufficient torque. As a result, the car will be “stuffy” before it reaches the red line speed.

Understand the relationship between gear ratio and tail speed

To find that “life size”, you have to understand the mathematical relationship between the front chainring and the rear (driven) chainring. The calculation of the tooth ratio is simple: divide the number of teeth before by the number of teeth after.

When you pursue tail speed, what you need is a smaller value (I. e. a more sparse gear ratio). By replacing larger front teeth or smaller rear teeth, you reduce the final gear ratio. This means that for every revolutions of the engine output shaft, the rear wheel rotates at a larger angle than the original setting. At high speeds, this allows the vehicle to cover a longer distance at lower speeds, theoretically increasing the terminal speed.

The Golden Rule Of Toothed Disc Adjustment”

When adjusting the tail speed, accuracy is the life gate. The standard in the circle is “first 1, top 3”.

Front gear plate adjustment: This belongs to “coarse adjustment”. Even if only one tooth is moved, the effect on the gear ratio is very significant. If you want to see the most obvious tail speed increase for the least amount of money, moving the front gear is usually the most cost-effective solution.

Rear gear adjustment: This is used to “fine tune. If you find that the gear ratio is too sparse and the power cannot keep up after adding front teeth, you can switch back to the original front teeth and reduce the number of rear teeth by 1 to 2. This will find a compromise point that will preserve the dynamic response and pull up the speed.

Horsepower Vs. Air Resistance

The reason why there is no one “perfect size” for everyone is because the laws of physics are there. As the vehicle speed increases, the air resistance increases exponentially. To run a higher tail speed, your engine must have enough “residual horsepower” to tear through the air wall.

According to my experience, many novices will change the gear ratio too aggressively (the rear teeth are too small or the front teeth are too large), and the car will slow down instead. The reason is simple: the engine was forcibly pulled out of its optimal power band. When the gear ratio is too low, the engine loses the mechanical advantage (torque) required to overcome the wind resistance in the highest gear, and the power cannot be used.

How To Judge “Tooth Ratio Is Too Sparse”

The most ideal gear size is to allow the engine to reach the aerodynamic limit at the moment, the speed is just at the output point of maximum horsepower. If the front teeth are exaggerated, you will find:

Acceleration is weak: the “push back feeling” of starting or mid-stage acceleration obviously disappears.

Can’t pull the red line: After hanging into the highest gear, even if you weld the throttle to death, the speed will stop there and will not rise.

Hidden danger of thermal attenuation: The engine will last for a long time under high load, and the working temperature will soar.

How To Choose Your Best Size?

To find the right size, I suggest starting with the “golden rule. Most players can start by adding front teeth. If after the change, your car can still easily pull to the red line at the highest grade, and you feel that the engine still has spare capacity, then you can continue to fine-tune and try to reduce 1 to 2 rear teeth.

In the final analysis, the best size is a mathematical “sweet spot”-where the torque curve of your engine can compete perfectly with the “wind wall” at high speed.

Author: Alex Reed

“Hi, I’m a performance tuning specialist with over a decade of experience in motorcycle drivetrain optimization, I’ve dedicated my career to finding the ‘sweet spot’ where mechanical engineering meets raw speed. My approach combines rigorous mathematical calculation with real-world testing to help riders break through performance plateaus.”