An overview video by T238 of their new CNC Steel 13:1 Gear Set & Shim Pack. As this is a guide on how to install a T238 CNC steel gear set for an airsoft AEG, it mentions that the gear set comes in three different specifications: 13:1 (high speed), 16:1 (medium speed), and 18:1 (low speed). They explain that shimming is necessary for the installation to prevent the gear from wobbling and demonstrates how to check the gear and add shims.

WYSHTECH Airsoft tests the 18:1 SR25 Gearset Wyshtech Edition made by Jing Wang... "These SR25 gearsets have the tappet plate pin in the correct position and has improved fps due to the better tappet timing.  They also run smoother than SHS/ROCKET/Siegetek SR25 gearsets."

ARES Airsoft release their series of upgrade gear sets. These are hardened steel mechanical gear sets and you can choose either having a high speed gear or high torque ones. Interestingly, they have some really high torque, double torque, and triple torque ones. Depending on your need, one these sets may just meet your needs.

Ask your retailers if they already these available.