MEDIUM DUTY - Model 337 Whether you are hauling a delivery or working construction, the new Model 337 has exactly what you need. This truck is designed to help you perform while lowering your operational costs with the quality and durability you have come to expect from Peterbilt. With a choice of the fuel efficient PACCAR PX-6 and PX-8 engines, in both truck and tractor configurations and with a GVW of 26,000 to 33,000 lbs., it can meet the demands of virtually any business and nearly every work application.