considering the mass of the planet and the mass of all living things on it, even if all living things tried to tilt the planet by moving, it would be like 100 ants trying to rock the Titanic. The respective speed and mass is just not enough. The only thing that can change the axis of the planet would be a falling asteroid of significant mass, significant changes in the magnetosphere or extreme movements in the planetary water mass (movements so extreme that a water mass at least half the size of the atlantic ocean would form waves and be displaced properly on the planet. There is no known natural method of doing such a thing except with falling asteroids who's mass would destroy the planet anyway...). Now, even if the axis tilted, then there would be extreme environmental changes but it would not be called global warming. Global warming is the increase of the planets temperature, which in this case is caused by the greenhouse phenomenon. Such phenomenon is caused by altering the ratios of greenhouse gases positively (increasing them). Such gases normally keep the earth warm by keeping solar radiation in the atmosphere but too much would cause overheating. You using your car less does help but the difference in your car use is a percent so small that would have practically no effect. Even if all people on the planet stopped using their cars the effect would not stop. The entire planet's cars only produce about 4-5% of the greenhouse gasses. Industry, power production, manufacturing, heating, mining, transport (all kinds), they produce all the greenhouse gasses. The residential sector as a hole only produces about 10% of the green house gasses, the rest is mostly in the industrial sector.
BTW, the earth was never flat. Pythagoras calculated the diameter of the SPHERE of earth back in ancient Greece and Galileo proved that the earth is round using the stars, plus that the sun is in the centre. Also, now we have satellites and we know it. Also no matter how you change an axis the shape never changes, just the rotation. And no flat planet can exist, at least not through natural means.
I hope I helped.