^ Thanks for your kind words, Tappet
Yes, the Tilix valve is good but if it's not set to provide near-full boost you'll actually lose power and have slightly worse economy.
Apart from that, it's an auto, and the torque converter (which is not a direct drive and thus loses engine revolutions) has a stall speed around 2400 to 2700 rpm - which means that it's at its most efficient around in that area, so if you keep the revs steady in this TC stall window as you're accelerating you will keep the drive train at its most efficient too, and use less fuel (I've tested this and it works).
The other important thing about the auto is the torque converter lockup (engagement of the torque converter clutch, or TCC). This locks the torque converter so that no revolutions are lost (increasing efficiency) but requires certain conditions to be met. It needs a certain speed, a specific gear, a certain engine load and everything running "within normal range" (temp of engine coolant, gearbox, that sort of thing). You'll notice the TCC engaging around 70km/h in top gear under light load - the engine RPM will drop without changing gear and the car will maintain speed. It will continue this way until certain conditions are broken - vehicle speed drops below 70km/h, engine load increases over 75%, gear is manually changed (this is how it works in mine, others might vary a little in numbers but should be close).
As for fuel consumption, are you filling the tank completely, and how much fuel are you putting in after driving 450km? If the full 80 litres (dangerous, because it lets the pump run dry) then your economy is 17.8 litres per hundred km.
I would suggest a couple of things, after filling the tank completely:
* Adjust your driving style for one tank, to accelerate within the TC stall window only
* Encourage the gearbox to engage the TCC by relaxing the throttle a little as the car passes through 70-75km/h (you'll spot your exact speed after a while)
* Try not to use full throttle at all
It would also be an advantage to grab an ELM327-compliant OBD2 bluetooth adapter and use an app like Ian Hawkin's Torque (Android) to monitor things like boost pressure (important). If you're not getting around 20psi boost during normal acceleration there might be something not quite right with turbocharger control (my own BCS has failed a couple of times, so I replaced it with the Tilix valve).