Hi Carbon - thanks for the kudos. It's a fun build. In reference to your curiosity there is a discussion about this subject early in the thread. Another reader noted that spider gears are not designed to handle this duty (which I agree with) and should be welded up. There are two ramifications of this: 1) I don't have to lock the other axle somehow (a good thing) as the Echo uses an open differential in the transmission, 2) the overall speed of the vehicle is reduced by 2 (not so good). I now calculate the top speed to be about 18 mph at 6000 rpm.I am curious about your brake steering system, especially since there was some mention of welding your differential spider gears.