Springs aren't the way to go and neither is compressed air. You need gunpowder!!
Seriously, aircraft used to use the Coffman starting system because it was far lighter than the rest.
Coffman engine starter - Wikipedia, the free encyclopedia
Of course the neighbors might complain about you starting your car for a late night beer run. Of course the police might have a thing or to to say about you starting your car this way too.
However for the weight and thus efficiency a Coffman style starter and a box of blank shot gun shells will weigh far less than any other method except for push starting.