The transmission is composed of a timing belt that connects a small pulley attached to the motor and a large pulley attached to one wheel.
A timing belt is a flexible and non-slipping mechanical drive belt with teeth molded onto its inner surface. Multiple types of timing belts are available but for this project, I use a 5mm Pitch HTD Timing Belt of 9mm wide. HTD belts have been designed to withstand loads.
The motors pulley is way smaller, with a small number of teeth (usually between 12 and 18 teeth). You have to be careful of the pulley bore diameter. For this project, I use a 12 teeth pulley 8mm bore.
This pulley is larger (with a higher number of teeth, usually between 35 and 40) and is attached to the wheel. I used a 36 teeth pulley that threaded through the cogs of the Abec 11 wheels with a large washer on the outside.