So you would like to build your own outdoor RC robot capable of going under vehicles and inside pipelines? The Oside Robotics Carbon Fiber 6WD Robotic Platform has you covered. It has hi-torque planetary gear motors. It has linear speed control with 3 speeds.
This platform has 2 x 5000mAh batteries. It has hard grippy tires that can be easily replaced. It has max speed of 7.3mph. You can find out more on RobotShop.