You can sound as smart as you would like axa. And yes, I know all about ratio math and numbers. I've built home made multi ratio power transmissions from scratch with belts and pulleys successfully ( I could send you pictures of my two speed belt driven motored bicycle that works like a charm). But the number of teeth on any said sprocket directly correlate to the circumference of same sprocket hence dictating what final ratio you end up with. All I was saying is, We noticed and was told by our local motorcycle mechanic when we were kids " you can add one tooth to your front (drive) sprocket or lose 4 on your rear to get the same (or very close to) end final ratio result. That's how we did it and it worked is all I can tell you. Now when you build a transmission from scratch , you have a lot more variables than what sprocket to use. Try having to figure out how much distance your drive tire OD covers per revolution and the list goes on. And I never said take 4 from the back "AND" add one to the front. I said do either or ! Not both !