After testing different P&G on the highway (60-100, 55-95, 55-90, 55-85, 55-80 and 60-80 km/h), I can definitely say that doing P&G on the highway is worse than doing constant DWL (Target Driving) with the Mazda. This can be explained by the fact that the accelerations are around ~20 LHK (80 LOD) and it takes a lot of time to accelerate to the targeted speed, this increase the AVG by ~0.3 LHK, then when I put it in neutral, it lowers the AVG until I reach ~6 LHK and then it stops lowering.
The car slow down so quickly that sometimes, the accelerations are longer than the coasts... I was going 90 km/h going downhill in neutral and the car slowed instead of staying at the same speed or accelerate. This is definitely not good, but this is caused by the automatic tranny and bad aero... The good point is that the better P&G was 55-80 km/h, this prooves that lower speed = lower consumption
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On my last tank, I did no P&G on the highway and P&G in town, the (fake) AVG that my sg reported was aproximatively between 5.0 LHK and 5.6 LHK. I got the same kind of cold weather (between -10°C and -25°C) but doing P&G in the highway portion of my trips dropped my LHK between 6.0 LHK and 6.6 LHK.
Update : This morning was warmer (-8°C when I quitted), c°WT showed 31°C when starting. EBH plugged 3 hours and 20 minutes. SG said 6 LHK for the trip.
On the returning trip, c°WT showed 10°C when starting. SG said I did 8.4 LHK in the first part of the trip (town) before going on the highway. After that I tested P&G and SG said 6 LHK at the end of the trip.
I will stop doing P&G on the highway and will do DWL instead.
This tank will be significantly lower than the last one, my brothers are now on the car insurrance and will drive the car around town to learn how to drive. I have a hard time getting under 10 LHK in town, so I guess that my brothers will do around 15! Hope to get this tank under 8 LHK.