What is the gas mileage on a dodge 2500 magnum 5.9 liter engine?

There's no Dodge 2500 Magnum with a 5.9L engine. The Dodge Magnum was a car, not a truck, and the 2500 series is a heavy-duty truck. The Magnum's engines were significantly smaller (typically V6 or V8, but nowhere near 5.9 liters). The Dodge Ram 2500, however, *did* offer a 5.9L engine in some years.

The gas mileage for a Dodge Ram 2500 with a 5.9L engine (typically a Cummins diesel) will vary greatly depending on the year, transmission, drivetrain (4x2 or 4x4), and driving conditions. You'd be looking at significantly lower fuel economy than a car; expect figures in the low teens (mpg) at best for combined city/highway driving, and considerably lower if you're towing or driving off-road. To get a precise estimate, you would need to specify the exact year and model of the Ram 2500.