Yeah I thought that, there's one of the build diaries where the builder talks about how you just need a lip to raise an opponent's wheels off the floor, and you can shove them around till your heart's content. Think it might be Jamie's. My beetle has 500rpm gear motors and will out-push two of the kinematic pay to drive robots at once. I know these are 3kg each give or take, so it just made me wonder. In feather circles I know that drills are standard, but drills at 7.2V still pushed opponents around in my first robot. So 7.2V x 7A gives about 50W per motor. The 37mm pololu stuff is cheap and 12V at 5A about 50 - 60W. There are then two ratios, one of which is pretty close to the 550rpm output of drills, and the other is 300rpm ish which on the 90mm wheels I'm planning might make it a bit slow. Gonna have a think for a couple of days. I'm leaning towards 300rpm as it's more controllable and arguably more torquey.