An aftermarket exhaust system can increase the horsepower of a 2500 series pickup truck by reducing back pressure and improving exhaust flow. When selecting a new exhaust, you should consider factors such as the material (e.g. stainless steel or aluminized steel), the design (e.g. cat-back or axle-back), and the manufacturer's reputation for quality and performance. It's also important to ensure that the new fumes system complies with local emissions regulations.