5.9 vs 6.7 Cummins: which is better for long-term ownership? It depends on what you do with the truck. The 5.9 common rail has a cleaner reliability record. Fewer emissions systems. Simpler turbo. No DPF. No EGR cleaning cycle. Repair costs are lower too. Turbo service is easier. Injectors are simpler to deal with. The 6.7 is not fragile, but the VGT can stick from soot, and head gasket trouble shows up more often on hard-working trucks. On the other hand, the 6.7 pulls better. More low-end torque. Better exhaust brake. Nicer on steep grades. With a G56, the 5.9 is easier on the clutch. The 6.7 hits harder down low, which can wear clutch parts faster. Fuel economy? The engine is only part of it. Cab size, axle ratio, tires, and driving style matter more. A clean history beats a better engine.