I bought my Prius last September. I noted that the stated fuel tank capacity was 11.9 gal and that when the fuel gauge started to blink, it would take about 9 gal to fill. I didn't have to strain too hard to calculate there is a 2.9 gal reserve. At 50+ miles per gallon, that meant around 150 miles.

Well, to my great surprise, I ran out of gas this morning on my way into work, 50 miles after the fuel gauge started to blink. Lucky for me I was just a couple miles from a gas station and made it before the battery was depleted. However, filling the tank only took 9.8 gallons! Doubting my sanity, I checked the capacity again on the back of the manual. Yep, 11.9 gal. Then I saw the note to "see page 320 for details". OK, maybe page 320 can enlighten me, I thought. Sure enough on that page was a blurb that read...

"The fuel tank capacity is decreased at low ambient temperature. (decreased by about 5L (1.3 gal., 1.1 Imp.gal.) at -10C(14F).)"

Holly smokes! That's almost 11%! What is going on here? Does anybody know why this happens? Doesn't this throw off your mileage calculations?

Example: You fill up when it's cold out, say it takes 9 gallons, then drive 450 miles on that tank. The next time you fill up, it's warm outside, so it takes 10 gallons because the tank expanded. You think you drove 450 miles on 10 gallons, when actually, you only used 9, right?