In addition to the sibling answer, CO2 is both a gas at any sensible pressure for a diesel tank, and has more molecules (and mass) than the constituent fuel. T

In addition to the sibling answer, CO2 is both a gas at any sensible pressure for a diesel tank, and has more molecules (and mass) than the constituent fuel. This adds up to more volume than the original diesel— a lot more. There's no actual efficiency to be gained here, especially since trucks are more constrained by mass than by volume.