I track fuel economy in my cars with an old Palm application (automobile by linkesoft.com). I've always noticed a definite seasonal fuel economy change in my Volvos, or at least, our former 850 NA and current XC90 2.5T.<p>Funny thing is, my V50 T5 is steady as a rock, seasonally. It doesn't seem to respond much to temperature or fuel formula. Now, it has a different fuel management system in the Bosch Motronic ME9 (vs ME7 in the XC90), but this really surprises me.<p> <IMG SRC="http://users.rcn.com/tmtalpey/V50/38934.jpg" BORDER="0"> <IMG SRC="http://users.rcn.com/tmtalpey/V50/38949.jpg" BORDER="0"> <p>As you can see, we've had the XC90 and V50 for about the same time, and although the XC90 has ~57K miles to the V50's ~43K, after the break-in period these graphs are completely different.<p>I don't have a good explanation for this yet, but it sure is interesting. Anyone else have a similar experience - or different one? Does the 2.4i behave differently, for example?<p>Tom.