Here in the UK we have been experiencing the slow onset of wintry weather along with earlier nightfalls and I have been noticing an interesting variation in my car's fuel consumption. That it is a Prius is probably not very important; but it does keep me advised on consumption figures, which have set me wondering.
When it was new in early August, I was getting 58-60mpg(UK). The weather was mainly warm and dry and most driving was in daylight. So equipment such as headlights and electric rear window demister were not used seriously. OTOH, the electric a/c ran often.
With temperatures down to around 3-4 C and rain on many days, the demister had heavy use, along with headlights, and mpg(UK) dipped to 54 or so. When the car is still cold, I think windscreen heat comes from the battery, which must be replaced (if indirectly) by the petrol engine, thus using fuel; this last is a guess, though.
Lower temperatures led to drier air, with waste engine heat being enough to demist. But mpg(UK) still went through a 50-ish period from which we seem to be recovering slowly (up to 52+).
The general driving style and routes have not varied much. Where they have varied, mpg have tended to go in the "wrong" direction: ie, up when they should go down, and _vice versa_.
Tyre pressures have been watched carefully. I have not driven in snow, or more than token coats of mud and/or water on the road.
This leads to two technical questions:
1) Typically, what sort of a load (in KW) do a full set of night- lights (headlights plus sidelights) plus the rear window demister cause in a medium-sized Toyota? (Prius, if it matters.)2) Assuming I go to the same garage and fill up with what is sold as the same basic grade of unleaded fuel, what percent variations in calorific value are likely? Would +/- 9% be improbable? I go to a big UK supermarket chain (Sainsburys) selling its own brand.
Am interested to hear thoughts on this.