now if i dig around my basement for a, uh, fortnight i can probably find the greenpeace report where they calculate that 30% of all co2 released by a car over it's entire life occur during its manufacture. now, those numbers were for the mid-90s, but the math and research that went into was pretty thorough and it's all outlined.
this actually harps back to the some thread a week or so ago where someone pointed out that a hummer's total emissions are actually less than a prius'. this is because the manufacturer-estimated lifespan of a hummer is waaay longer than a prius, so that manufacturing pollution gets spread out over a longer time.
not sure if that stat would help you anyway. after all, the objective is not to get people to go car free, after all.