The CTD BS meter accuracy varies widely in stock form. Best ever empty with stock ECM program was 19.8 mpg, overhead said 23.5 mpg. (the overhead was only reset at each fillup). Normal highway is around 18 so that 19.8 was probably a short fill up. Some have posted their overheads read lower than actual mpg. The only way to know is to check it over several tanks.
A Dodge tech posted this on another forum:
The ECM rcvs inputs from sensors,and knowing fuel pulse widths,Injector flow #s rpm,map tables etc computes the #s.There are no flow or fuel psi sensors involved.
It is simple to understand why a aftermarket box will screw up the mpg accuracy, the ECM simply does not know that the box has held the injector open longer or raised the rail pressure allowing more fuel to go into the engine, so the ECM thinks less fuel was injected and displays higher mpg than actual. When you have a performance box any hope for accuracy is gone.