Here's my theory on this topic.... my t-7 car somedays shows more boost than others. On my morning commute if i drive slow, and dont floor it anywhere my gas mileage goes up obviously. when i get outta work i sometimes hit the gas a bit harder. when i leave work is where i notice the most difference, my boost is lower i hit like 2psi less. If its a weekend where i usually cruise around and have fun i get more boost almost like the ecu has reprogrammed the timing and thus giving me better spark and more boost.
I know T-7 engines adapt but I'm not sure what engine you have or if T-5 adapts in a similar fashion. I have never heard of adapting down but it only makes sense. Adapting up is done by driving the car in 3rd gear at full throttle for basically the whole power band. I can't remember the exact time recommended but the point is to make the engine think you want to drive like a bat out of hell all of the time so it will provide more power when on full throttle.
the reason for increased gas mileage is your newfound driving style. the shift light inside your car is made to increase gas mileage by telling you to shift up to 5th gear or 6th gear as soon as possible. For instance, if you shift up to 5th gear and are at 2000 rpms, you'll get far more mpg than if you are in 3rd gear @ 4500 rpms at the same speed.
ERP has instructions on his website on how to convert that in dash shift light into an actual shift light for most torque not most mpg
Ultimately, assuming that fuel ratio, and ignition timing don't change ...
... fuel mileage is going to be based on whatever loss is due to friction on moving parts in the engine.
So ... lower revs, but more throttle, means same accelleration (same time spent accellerating) but less friction because more of that fuel goes into each individual revolution, so since you turn less total revs to get the same power, you waste less energy.
There have been a lot of studies on this actually. Turns out the most efficient engine is one which runs a 16-17:1 a/f ratio, Maximum ignition timing for best torque, and full throttle.
To make world records on fuel mileage they would actually accellerate at full throttle and then shut the engine down, then start it and run full throttle and shut it down again.
To simulate that in a car, you'd drive at the maximum throttle angle that still maintains close to stoich a/f ratio, keep the RPM as low as possible, then coast in the highest gear available at the time for as long as you can before stopping.
The Viggen actually stays close to MBT and 14:1 even at full throttle if you stay below 3,500 RPM ... same with most newer Saabs. That means if you keep the revs low, you can get very good mileage even if you drive hard.
No, really, you should change two things in your driving style. Don´t coast, use lower gears and step of the gas. Using the engine to brake. When the throttle is closed and the rpm is above 2500 your engine uses NO gas at all. This is due to "fuel cut off" when the throttle is closed. All newer SAABs have this.
Also, use your Cruise Control. Unless it´s hillly, the CC is very good at keeping the MPG up.
Now, my best MPG (or, I see it in litres/100km) is 42.8MPG or 5.5litres/100km, which is very good for a car with 235hp