There really isn't any "gearing" so to speak as the cab assembly will drive the carriage assembly through the blade via flat belt drives. Speed of the carriage will be sped up or slowed down by how much pressure is applied to the forward/reverser. The flat belts allow the carriage to literally creep through the blade with a large log or speed quite quickly through a smaller log.
I would be glad to take some detailed pics of a couple of mills I have here showing how different they can be. One is a FoleyBelsaw with a pto drive and a 50" blade and the other is a home made mill, pieced together with a drive mechanism that is quite unique. As a matter of fact there is a post titled 40' sawmill on a 20' trailer in the machinery forum you may be able to get some ideas from. Let me know if you'd like some pictures. By the way, I've traded for another mill and have to pick it up sometime this winter. I'll take a ton of photos too.
I think your biggest issue will be in the carriage indexing mechanics. It will have to draw the log forward across the carriage to line up with the blade, but will also have to be able to be ratcheted back to the other side again. A reliable way to " dog " the logs to the carriage could be a bit of a problem too, but with a little ingenuity done well.