Fuel mileage of vehicles equipped with a 6.2 or 6.5 diesel engine can vary quite a bit depending on vehicle health, transmission options, gearing and driving habits. Speed is more critical on a diesel than a gas engine. On the highway, in the 50-75 mph range, the fuel economy will go down about 3 mpg for each 10 mph increase in speed. This condition is perhaps the most significant factor in obtaining good fuel ecomomy. Fuel economy may vary as much as 5 mpg in a given vehicle with different drivers.
Diesel engines have about half the useable rpm range as compared to a gas engine.
A gasoline engine will produce acceptable fuel economy over a fairly wide rpm range. The diesel engine has a relatively narrow rpm range that will produce acceptable fuel economy. The relationship of gearing to fuel mileage has to be the single most misunderstood aspect of the GM diesel engine. A typical 6.5TD with 3.08 or 3.42 gearing will get in excess of 20 mpg at 65 mph because the engine rpm at that speed is right at the engine's torque peak of about 1700-1800 rpm. A truck with 4.10 gearing is running at about 2250 rpm at the same speed and typically gets 15 mpg.