I know that 400 km/h is a football field per second

So adding the drag that would reduce the speed, altough a high speed is still possible.
Since is hard to find a very long non-busy road so the rocket could go from zero to zero and since launching a high-speed projectile in the ground could be dangerous I think it would be better to limit the rocket´s volume so it would run fast but not by to long.
That measure reminds of the Bugatti Veyron, its wheels allows it to stay at 400 km/h for 15 minutes...but for security the fuel tank has fuel only for 13 minutes at that speed.
It´s needless to say that any form of breaking mecanism is useless...no one has a reaction time to activate it before the rocket passes 40 meters at 200 km/h.
Conclusion:
- What is the minimal volume for a WR-car speed record ?
- What is the minimal distance accepted for a WR-car speed record ?
- How the speed record will be measured ? By a radar-speed-gun-toy ?