This would be possible in theory.
In order to just complete the loop, without falling off, the force due to gravity at the
top of the loop would have to just equal the centripetal force on the car as it followed
the path of the loop.
So, for a circular loop:
mg = m(v^2)/r
where, m is the mass of the car, g is the acceleration due to gravity,
v is the speed of the car, and r the radius of the loop.
Note that, therefore, the m's cancel:
g = (v^2)/r
i.e. the criteria for successful looping do NOT include the mass of the car (but only its
speed, and the radius of the loop)
Assuming a top speed, say, of 100mph, then, looping could be accomplished for a loop radius
of about 200 metres, or less.
I still wouldn't like to try it!