And here is it: big and faster than you probably think...
To get better control in long range I have used SBrick.
The overall size is 50x30x21 studs (~40x24x17cm). The wheels used the ref no. 92912, from the Unimog set.
To do it as fast as possible I have built it very simple and light, it only has 698 parts, and the weight is only 1.3 kgs.
It is powered by 4XL motors, 2 for each axle with 1:3 final gear ratio. With so many motors it needs to use 2 batteries and 2 SBricks.
The max speed is around 8kph (5 mph).
I have used a special configuration for the motors to get the best possible preformance.
With it the transmission is very simple and it only has 4 gears... and nothing more. Simple and effective.
The suspension configuration is a double wishbone in both axles.
It is every soft and with long travel, essential to go fast on bad terrain.
The steering are controlled by a PF Servo motor and working steering wheel
Available Free LDD Instructions
Download it HERE (downloaded times)
Finally as always a complete video in action: