The 6.2L and the 6.5TD longblocks are basically the same, with the exception of bore diameter and precups.

Assuming 3.73 gears, a 6.2 powered truck can deliver low 20's for fuel economy. With the same gearing, a 6.5 equipped truck can produce high teens. There could be 5-mpg difference. Some or most of which is, I believe, attributable to the precups.

GM once sold a 6.5L diesel engine named the "Fuel Miser". The only difference between that engine and a typical 6.5 is that the Miser used smaller port precups and just the rear cylinders received oil spray.

Jim