Originally Posted by cssnms
They are both piggybacks and I do believe the VAC and JBD operate and make power in a very similar manner, in that they both intercept the signal from the ECU to the high pressure rail sensor, in short they are signal benders. The only way to effectively modify injection timing is by reprogramming the ecu. And yes, your theory regarding skewing the computer readings is correct, but that theory applies to both. If your ecu is being told not enough fuel is being delivered then your computer is reading that your consumption is lower. Diesels make extra power by injecting more fuel which is accomplished either through increasing injection timing or increasing fuel pump pressure; more fuel being delivered translates into more power, which more often then not yields lower mpg's. That said under certain circumstances you are likely to experience better than avg fuel economy simply because your engine doesn't have to work as hard to maintain speed since it is makeing more power lower in the rpm band, which I think will make itself more evident during highway traveling.
The computer learns and makes adjustments, btw. When I first got my JBD, I got some crazy readings for my MPG and my distance to empty (talking it showing I was going to get 800 miles on my tank). The more I drove, the more it stabilized.