MPG in hybrids are very sensitive to weather extreme conditions, in my experience the best MPG are in middle season weather, where there is no need for air conditioning, HV battery works at ideal temperature and thermal engine reaches relatively fast its working conditions; between winter and summer winter is worse, may be that in warmer climate zones the results are slightly different, as I am considering summer season with temperature over 30° C (and consequently AC on) and winter just around 0° C. Generally speaking, I found best results driving with dry weather at about 22° C. Another great difference is the lenght of journey, as short ones need to warm up thermal engine more time s and in this phase is used much more gasoline (it is the same for traditional cars too).