Vehicle Mod Laws in Montana

Replacing the exhaust system is legal in Montana, but a muffler in good working order is required and cutouts or bypasses are prohibited. There is no statutory 'no louder than stock' rule — the standard is that the exhaust must not produce excessive or unusual noise.

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Montana caps exhaust noise at 95 dB for all motor vehicles except motorcycles and quadricycles, measured under SAE J1169. A muffler in good working order is separately required and cutouts are prohibited.

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Smoked or tinted film on headlights and taillights is illegal in Montana. Headlights must emit white light and taillights must show red at the required visibility distance, and tint that dims or discolors them violates those requirements.

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Frames are legal only if the plate is not obstructed — it must be conspicuously displayed and may not be obstructed from plain view, which protects the whole plate face including the state name, county number, and decal.

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Montana sets no statewide lift, frame-height, or bumper-height limit. The only cap is overall vehicle height of 14 feet, and general equipment rules — headlamp height, working lighting — still apply.

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Montana requires no cover. Off-road lamps mounted above 42 inches simply may not be illuminated while operating or parked on a highway, and no more than four front lamps over 300 candlepower may be lit at once.

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Montana has no statute limiting how low a vehicle may be. General equipment rules — headlamp height, working lighting, nothing dragging on the road — still apply.

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Underglow is legal in Montana, including while driving, subject to color and flashing limits: No blue (emergency only), no red to the front, no rotating or flashing, and beams over 300 candlepower aimed within 75 feet.

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Montana requires fenders, splash aprons, or flaps on the rearmost wheels of every vehicle except motorcycles, quadricycles, motor-driven cycles, and farm tractors — but sets no limit on how far a tire may protrude.

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No sign, poster, or non-transparent material may obstruct vision through the windshield, and sunscreening must be clear and transparent below the AS-1 line — confining any strip to the area above that line. Because the material must be transparent, a printed or opaque banner is not legal in Montana — only a clear tint band.

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Front side windows: minimum 24% VLT. Rear side and back windows: minimum 14% VLT. Darker limits apply behind the driver on some larger vehicles.

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