This year we developed and used a hybrid of an octane. we went with six wheels instead of 8. We used 4 2-stage vex transmissions geared 3:1 and 8:1. We also used a 12 tooth sprocket from the transmissions to mecanem wheels, and then a 22 tooth sprocket on the mecanem. We then moved to a 44 tooth sprocket on the 6 inch wheels. We built two pods for the mecanem and traction wheels duos, and then ran a bar connected to each beneath the central frame of the robot. A 3 inch piston pushed the bar down in the middle of the robot, engaging the traction wheels and lifting the other mecanems in the pod off the ground.
See link for a picture.
http://www.chiefdelphi.com/media/photos/40492
We welded the pods using 1/4 inch bar aluminum. Our frame is made of c-channel. It was not the lightest of drivetrains, but we still fit everything else on the robot with not too much worry about weight.
We could push 90% of the robots on the field, and we could hold our ground against the other 10%.