Re: Great weather = great FE!
I have seen the same thing too, and the difference is bigger than expected. Even smaller steps in temperature makes big difference in FE. I am more and more convinced that it has to do with the strategy of the engine management. At 10 deg C the car is less prone to shut off the engine when coasting. Going downhill at around 70 km/h the instant FE display will show a few bars (around 2 L/100 km), sometimes together with one or two bars of regen, unless you let go off the pedal completely. At 20 deg C the engine shuts down long before you release the pedal completely. Also in the warmup phase there is difference. At 20 deg C auto-stop is enabled more or less directly from start (cold engine), but at 10 deg C the engine has to reach almost full temperature first. I guess this strategy is primarily to get heat for the climate control (and not so much to reduce wear on the engine).
My own estimate is that FE is 10% better at 20 C than at 10 C, also for longer trips. I haven't done any serious testing or measurement, though.
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