20 inches? Jeep would fail... Far too close for the turndown to have any effect.http://www.maine.gov/dps/msp/vehicles_inspections/documents/MaineMotorVehicleInspectionManualRev121506_000.pdf
The test is-
(SAE J1169)- SEMA law... 95db nationwide limit-
Under the SAE standard, a sound meter is placed 20 inches from the exhaust outlet at a 45 degree angle and the engine is revved to three quarters of maximum rated horsepower. The highest decibel reading is then recorded.