It seems to me that your driving style would make more of an impact than what you drive. The guy who owns a pickup with a V8 but only drives 3000 km per year hurts the environment less than the guy who owns an economy car but drives 30,000 kms. Also things like how hard you drive matter, an economy car beign driven at full throttle would use as much gas and spew as much toxins as a V8 being driven very lightly. It's far more complicated than just what you drive.