Last summer, 2011, or what little summer we actually enjoyed here in Scotland, the Prius was averaging around 54 to 55 mpg. This was all in my usual routine of mostly urban driving (school run, shopping, etc.) with about a third on country roads (travelling between villages on a mixture of A- and B-roads). The journeys were mostly completed by me and two small children. Tyre pressures were set to a slightly higher 41psi to reduce rolling resistance and have been maintained at this throughout, save for a drop in pressure of 1 or 2 psi between checks occasionally.
I changed the aerodynamic efficiency of the car later in the summer when I fitted Thule roof bars (a Thule 754 foot pack with Thule square bars). Economy subsequently dropped to around 52 to 53 mpg in the same circumstances. Now in the wintertime, and experiencing the coldest temperatures of the winter season so far, the Prius is only averaging about 50 mpg. Daytime temperatures have been between -1°C to +4°C. Yes, I have kept the roof bars on, and yes, I should take them off but I haven't done so for a couple of reasons I won't go into here. However, it all got me to thinking that I should consider recording on a weekly basis what I am experiencing in terms of mpg, tyre pressures, type of driving, average temperature range, etc. It might make an interesting set of charts and graphs over time. Well, interesting to me anyway. I can hear my wife sigh now just at the mere hint of spreadsheets and graphs!