After the launch and six-hour cruise, the Falcon Heavy’s upper stage will fire a third time to send the Tesla into a cycling orbit between Earth and Mars. This should take the Tesla out as far as the Martian orbit around the Sun, or even a bit further. Musk said the vehicle should get as far as 380 to 450 million km from Earth, depending on how the third burn goes. ( source )
80 million kilometers is a huge tolerance. Normally rockets make very precise burns to arrive at specific trajectories and how the amount of fuel carried converts to delta-V and possible trajectory is well known.
What is the cause why the planned trajectory was defined with such poor precision? While I understand they are not aiming at any specific target, they should still know how much delta-V the rocket carries and how far it will get it.
Like, I'm happy your rocket can reach anywhere between Mars and the Asteroid Belt, but I'd be more confident if the word "anywhere" wasn't applied in so literal meaning...