The reason Ford says the new gear set is not for emission controlled cars is because it will slightly affect emissions. Putting on a lower gear set will improve times off the line by putting you in the power band quicker. It also lowers fuel milage because RPM will increase - which is what affects emissions. Lower gears result in higher RPM's to maintain the same speed. So if your car runs at 2500 RPM at 70 MPH stock, you may run 2800 RPM at 70 MPH with the lower gear kit. Higher RPM's mean more fuel consumption -- higher fuel consumption means increased emissions. You will see quicker times off the line with the gears, but the car will be a bit louder at speed because of the higher RPM's and fuel consuption will drop a bit. Hope that helps -- the real pro's and con's of a gear swap. Better to make the decision with all info up front so you are not disappointed with the results.
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99 Mustang 3.8L, C&L mass-air system, GT dual exhust and rear bumper, Maximum Motorsports 4-point K frame brace, full length sub frame connectors and strut tower brace.
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