I haven't been able to find any statements on the exact target orbit, but the general consensus is that the payload won't be in orbit around Mars, it will be in an eliptical orbit around the sun and 'touch' the area of space that Mars orbits within. Reaching an orbit around Mars requires much more fuel, the real goal of this launch is to test the rockets capabilities so it makes sense to be able to use some fuel to land the boosters rather than use that fuel to insert a rather useless payload into Mars orbit.
As suggested in the comments below, Musk may have been purposefully vague about the target orbit in an attempt to make us assume it will be in orbit around Mars, and therefore a grander achievement than it really is. The SpaceX website makes the same statement, claiming that Falcon Heavy can take a 16,800kg payload 'to Mars'. The weight of a Tesla is around 1,300kg.
From Musk's Instagram post:
Test flights of new rockets usually contain mass simulators in the form of concrete or steel blocks. That seemed extremely boring.
Of course, anything boring is terrible, especially companies, so we decided to send something unusual, something that made us feel.
The payload will be an original Tesla Roadster, playing Space Oddity, on a billion year elliptic Mars orbit.
The phrase "billion year elliptic Mars orbit" sounds like it could be an elliptical Mars orbit, (orbiting around Mars) in a technical context, but in a social media post, the interpretation could be a bit more loose.
To explain the expected orbit, the image below is a diagram of a Hohmann Transfer, the most fuel efficient method of transfering between two objects that orbit the same body.
The grey circle is the Sun, the green ring is Earths orbit around the sun, and the red ring is Mars's orbit around the sun. The transfer is done with two burns, first you move from the green orbit to the red orbit, then once you reach the target orbit, you circularize your orbit.
SpaceX plans to make only the first burn, and orbit in the yellow ring, allowing them to reach the orbit of Mars around the Sun, but not stay there forever. The 'Half-Hohmann Transfer' allows them to do this as cheaply possible and still claim to reach Mars.
The tweet is a little misleading, rocket science is hard enough without the 140 character limit per tweet.