The Guide to Legal Car Modifications in Missouri

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If you live in Missouri and want to modify your vehicle, or if you are moving to the state with a car or truck that you have customized, it is important that you know the laws to make sure your vehicle is street legal. The following are the most important regulations for keeping your vehicle within the laws in Missouri.

Sounds and noise

The following are the laws regarding vehicle sound systems and mufflers in Missouri.

Sound system

Missouri does not have specific guidelines on sound systems other than that the noise emitting from a vehicle cannot be considered a nuisance or injurious to the welfare or health of the people who live within city limits or within a half mile of city limits.

Mufflers

  • Mufflers are required on all vehicles, must work properly, and must prevent unusual or excessive noise.

  • Muffler cutouts are not permitted.

  • Any muffler openings present must be attached in such a way that they cannot be turned on or opened while vehicle is in motion.

Tip: Also check with local county laws in Missouri to make sure you are following any municipal noise ordinances that may be more stringent than state-level laws.

Frame and suspension

Missouri does not have any restrictions on frame height or suspension lifts, but it does restrict bumper height.

  • GVWR under 4,501 – Front bumper height maximum is 24 inches, rear is 26 inches.
  • GVWR 4,501-7,500 – Front bumper height maximum is 27 inches, rear is 29 inches.
  • GVWR 7,501-9,000 – Front bumper height maximum is 28 inches, rear is 30 inches.
  • GVWR 9,002-11,500 – Front bumper height maximum is 29 inches, rear is 31 inches.

Engine

Missouri does not currently list engine modification or swap regulations. However, St. Charles, St. Louis, Franklin and Jefferson Counties require emissions testing.

Lighting and windows

Lights

  • Three auxiliary lamps are permitted on the front, mounted between 12 inches and 42 inches.

  • White lights are required for illuminating license plates.

  • Two fender or side cowl lamps are permitted that emits yellow or white light.

  • One running board courtesy lamp is permitted that emits yellow or white light.

  • One spotlight is permitted that does not glare on or dazzle another person.

Window tinting

  • Non-reflective tint is permitted above the AS-1 line provided by the manufacturer.
  • Front side windows must allow over 35% of light through.
  • Back side and rear window can have any darkness.
  • Front side and back side window reflective tint cannot reflect more than 35%.
  • Side mirrors are required if back window is tinted.

Antique/classic car modifications

Missouri cars can be listed as historic if they are 25 years or older. Vehicles with historic plates:

  • Have no mileage limitations when driven to and from educational or exhibition events.
  • Can be driven to repair locations within 100 miles.
  • Have a 1,000 mile per year limit for personal use.

If you want to make sure your modifications are within Missouri laws, YourMechanic can provide the mobile mechanics to help you install the new parts. You can also ask our mechanics what modifications may be best suited to your car using our free online Q&A system, Ask a Mechanic.


The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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