🚀 go-pugleaf

    I don't think that simply telling you the best and
quickest way to measure the mass of the car would really
be fair to the other contestents.  (Not to mention that
doing so would undermine the purpose of the competition
which is to give you an opportunity to attack a complicated
problem and create your own solution.)
    
    With that said, I would be perfectly willing to give
some advice.  You should probably be thinking about what
you actually measure when you are finding the mass of
something.  For example, how does your average bathroom
scale work?  What does it measure?  You said that you
couldn't move the car, but is there anything about the
car itself that could serve as a kind of scale?       

    On a practical side...  Once you have a method in
mind, I would practice, both on a real car and on    
something similar (maybe a bicycle with a person on
it for extra mass) that you could actually know the
true mass of by some other method.  Also, I would 
have any equations that you might need ready ahead
of time, with any unit conversions already figured
out, so that all you will need to do is plug in 
your measurements.

    Let us know how it turns out.

                                          - Dan