Pretty new to Blender and have been struggling with even getting this to work, but now it does. A big gear is parent to a smaller gear and, via drivers, I have them both spinning the same direction and at same speed. Thing is, I want the small one to spin about 3.5x faster than the big one. Of course, this is just part of my project, but one step at a time.
I followed the YT below as to how I got to this point. Start @ 2:44. I had to slow it down big time and pause it, rewind, etc. Days later, I'm to the point where I am trying to vary the smaller gear's rotation speed.
My big gear is 50t and smaller one 14, and I have a chain to connect them. My objective is to have it all in sync. In case it matters, its supposed to be a motorcycle chain and sprockets.
Here is one of my feeble attempts at getting the big gear to drive the little one. Rotate the axis in center...they both turn.
Dang it! I can't post a link with what I hoped the finished product would be like...but its like Paul's response..just my gears and a motorcycle chain.