Ummmm...how do you know that they all haven't worked mpg out properly ?
I don't and I wasn't making any reference to whether or not a remap is effective. It was the "It may be more as I suspect the Alfa are calibrated using US gallons" that I was commenting on, although now I feel like I've been a bit mean about it. If the guy is happy, he's happy. Sorry for my comments there.
Back to the subject though: stick a remapped vehicle on a dyno and you'll get a concrete result. On the other hand, vehicle efficiency is notoriously difficult to measure. I suppose 'tank to tank' measurements are the best we have but that's a far cry from controlled conditions.
I've always taken the view that manufacturers have targets on power that will get reigned in so they can use less durable components. Nothing revolutionary there, and many are willing to expose themselves to a controlled risk by opening up the envelope.
I have to admit that I struggled to extend this concept to efficiency, until I thought back to second year thermodynamics and the Diesel cycle. This depends on the cut-off ratio (fixed by the engine geometry) and the compression. You can see where I'm going with this... the effective compression ratio must have to increase, right? Surely that can only mean applying boost for which the engine was not designed.
Efficiency doesn't just mean quantity of Diesel used: it's also a fixed amount of CO2 out of the back per mile. Car companies are desperate
to get that number down, but obviously they have warranties on turbos etc. to worry about. So they compromise based on the durability of components.
There's an overarching opinion that a "remap for economy" is generally a "safe" thing to do. Having thought about it carefully, I don't really think so.
I must stress I am no authority on the subject, just a physicist and engineer who has previously considered remapping, read around and decided against it.
(good page here)
The Diesel Engine