NCY 15T / 37T Performance Up Gear Set for GY6 125cc–200cc Engines

Increase your scooter's top speed and make better use of your engine's power with the NCY 15T/37T Performance Final Drive Gear Set. Manufactured in Taiwan using premium materials and precision machining, this transmission upgrade is designed for riders who have installed performance modifications such as big bore kits, stroker cranks, upgraded camshafts, or high-performance CVT components.

Compared to the factory 13T/40T gear set, the NCY 15T/37T gears provide approximately a 20% taller (up gear) ratio, allowing the engine to maintain lower RPM at cruising speeds while increasing the scooter's overall top-end speed.

Why Upgrade Your Final Drive Gears?

The final drive gears are the last reduction gears inside your GY6 transmission before power reaches the rear wheel. Changing these gears has a major effect on how your scooter accelerates and how fast it can travel.

Stock Gear Ratio (13T / 40T):

• 40 ÷ 13 = 3.076:1

NCY Up Gear Ratio (15T / 37T):

• 37 ÷ 15 = 2.466:1

Because the numerical ratio is lower, the rear wheel turns farther with each engine revolution. This results in:

• Higher potential top speed

• Lower cruising RPM

• More usable power at higher speeds

• Better performance from engines producing additional horsepower

The NCY 15T/37T gear set provides approximately a 20% taller final drive ratio than stock.

Understanding GY6 Gear Ratios

Higher Numerical Ratio (Example: 3.07:1 Stock)

A larger gear ratio gives the rear wheel more mechanical leverage.

Advantages:

• Stronger acceleration

• Better hill climbing

• Faster take-off

• Ideal for stock or lightly modified engines

Disadvantages:

• Higher engine RPM at cruising speed

• Lower maximum speed

Lower Numerical Ratio (Example: 2.46:1 NCY Up Gear)

A smaller numerical ratio reduces mechanical leverage but allows the rear wheel to rotate farther with each engine revolution.

Advantages:

• Higher top speed

• Lower cruising RPM

• Better use of high horsepower engines

• Improved highway performance

Disadvantages:

• Slightly reduced mechanical torque at very low speeds

• Works best on engines that produce additional power

Which Gear Ratio Works Best?

Stock 125cc–150cc Engines

Recommended:

• Factory gearing

• Small up gears only if mostly flat riding

Reason:
Stock engines generally do not produce enough power to fully utilize aggressive taller gearing.

155cc–169cc Big Bore Builds

Ideal Choice:
NCY 15T / 37T

Excellent combination of:

• Strong acceleration

• Increased top speed

• Comfortable cruising RPM

Perfect for:

• Mild performance builds

• Camshaft upgrades

• Performance exhaust

• Variator upgrades

171cc–180cc Performance Builds

Recommended:
NCY 15T / 37T

The additional displacement easily pulls the taller gearing while maintaining excellent acceleration.

Ideal with:

• 61mm cylinders

• 63mm cylinders

• Stroker crankshafts

• Performance CVT systems

180cc–200cc+ High Performance Builds

The NCY 15T/37T is an excellent all-around choice, especially for street riding.

Engines producing substantial horsepower benefit from taller gearing by:

• Reducing unnecessary RPM

• Improving sustained high-speed cruising

• Better utilizing available horsepower

Specifications

Front Counter Shaft

• Teeth: 15

• Overall Length: 60mm

• Spline Length: 24mm

• Outside Diameter: 31.1mm

Gear Press-On Area

• Length: 12.08mm

• Outside Diameter: 19.08mm

Ends

• Length: 9–10.5mm

• Outside Diameter: 15mm

Rear Final Drive Gear

• Teeth: 37

• Spline Count: 19

• Outside Diameter: 71.2mm

• Inside Diameter: 18mm

• Width: 20mm

Installation Note

The factory countershaft gear is press-fitted onto the shaft.

Installation requires access to a 12-ton hydraulic press (or equivalent) to safely remove the original gear and install the new NCY gear.

Professional installation is recommended if proper equipment is unavailable.

Compatibility

Fits most GY6-based 125cc–200cc engines including:

• 157QMJ

• 152QMI

• 161QMK

• 1P61QMK

• YN1P57QMJ

• JJ157QMJ

• BN157QMJ

• BN161QMK-A

• 154FMI

• 1P52FMI

• 1P57QMJ

Also compatible with many other engines whose engine code contains QM, QMK, QMI, or FM.

About NCY

NCY has been producing premium scooter performance components in Taiwan since 1997 and is recognized worldwide for manufacturing high-quality drivetrain, suspension, engine, and performance parts. Every component is built to strict quality standards for reliability, durability, and long service life.