The advent of changes to London's congestion charge that makes the fees based on a vehicle's carbon dioxide emissions has prompted many automakers to tweak some of their models to get their emissions down. Any car that emits less than 120g/km of carbon dioxide is exempt from having to pay the charge. The latest brand to go after that threshold is BMW with their 318d. The existing version is rated at 123g/km, so BMW will make some changes to get into the charge free band. BMW engineers feel they can meet the challenge with the 318d. The smaller 118d already achieves 119g/km with it's start-stop system and brake energy regeneration. Both cars use a 1.8L four cylinder turbodiesel. Some changes in gearing, lower rolling resistance tires and some aero tweaks may be all it takes to meet the target.