I love the gearboxes and axles added to GT6, it really is the perfect combination of practical and “cool” in my mind. My support question (or perhaps suggestion?) is all about the topic title: Custom gearboxes only appear to accept full-size gears, not small gears; therefore, is it impossible to …

You CAN combine with the Big gears, but you have to have the smaller Turbines run at exactly the same Speed in order for this to work.