Car mods that are illegal in many states include straight piping your exhaust, using muffler cutouts or bypass valves, or installing gutted mufflers.
Customizing your car is a great way to make it feel more unique and true to your style. However, factory vehicle design is tightly regulated, and thus many car modifications are not street legal. For example, if you live in one of the very strict states that crack down on window tint, you could get a ticket if your car windows are too dark. It goes beyond that — if you modify your exhaust to make it louder or change your car’s emissions profile, you could be violating motor vehicle law as well.
According to World Population Review’s 2025 overview of U.S. state exhaust laws, the vast majority of states prohibit mufflers that create «excessive or unusual noise», as well as any mod that increases exhaust volume beyond factory-certified levels. Certain states such as California, Maine, and Montana use a decibel-based scale. Therefore, a modified exhaust louder than 95 dBA is illegal. Most others rely on a broader definition of loudness, without requiring formal decibel-based testing.
In practice, this often boils down to the «discretion» of the police officer, meaning that if they deem your exhaust to be too loud, it can get you into trouble. Granted, the standard is also upheld by many courts through the «reasonable person» principle, meaning an exhaust can be cited if it would unreasonably disturb an average member of the community.
