(10/10/12 edit: Moved running draft text formerly in post #10 to this post. I do welcome feedback about the content or any other links that should be included.) Since it's getting colder and there's even snow in some parts of the US already, no doubt people will experience the usual mileage drop during winter/colder weather. This affects all vehicles, not just the Prius. When the weather gets warmer, mileage complaints go down, as mileage rises in warmer weather. Here are some links discussing the reasons why: Why does mileage drop in winter? — Autoblog Green Cold Weather Vehicle Fuel Mileage – Why Winter Fuel Economy Drops – Fuel Mileage Drop in Vehicles Car Talk http://web.archive.org/web/200912070.../11354696.html Why does it get worse MPG in cold weather? In addition to factors winter/colder weather that directly affect a car's operation, there may be more delays and more travel time at inefficient speeds due to accidents and weather conditions. Those in cold climates w/o a garage or covered parking may run their engine w/the heater and defrost on high to help melt snow and ice off windshields. Fuel consumption is high at this point since the engine is still in warm up mode and yielding 0 mpg for the entire duration. Due to the Prius' typically high baseline mpg figure, the mileage drops may seem worse than they really are. To quote from Car and Driver: Mileage? No, it's Your Gallonage that Really Counts Here are some sample calculations to illustrate the above: 100 miles / 50 mpg = 2 gallons used 100 miles / 45 mpg = 2.22 gallons used 100 miles / 20 mpg = 5 gallons used 100 miles / 18 mpg = 5.56 gallons used So, if you had a 20 mpg vehicle that dropped to 18 mpg, you might not even notice it or dismiss it as noise. Yet, the 2 mpg drop in that vehicle actually caused ~2.5x more consumption. What can you do? Stuff you can do to improve your winter mileage (2010 Prius Grill Blocking strategy | PriusChat has another table of grille blocking suggestions for 2nd and 3rd gen Prius grille blocking) Please list your tips for better MPG in the winter. | PriusChat has some good hints although some are 3rd gen (2010+ model year) specific such as the mentions of HSI, likely #6 (the 46 mph that mentioned is likely the higher speed at which the 3rd gen can run in electric only) and blocking the lower grill (on 2nd gen, one should block upper grill first). Better heater management (more details at Which one? | Page 2 | PriusChat): if you're using the heater and while stopped, you find that the ICE (internal combustion engine) seems to run or start up "unnecessarily" when the HV battery doesn't appear "low", do this: - stop using auto mode on the HVAC system - turn the HVAC fan to OFF when you find the ICE seems to be running "unnecessarily" (it will run to provide requested cabin heat if the fan is on and the ICE is too cold, wasting fuel) - only turn the fan back on when the ICE must run anyway (e.g. hard enough accelerations or traveling >40 mph on the 2nd gen Prius and >46 mph on the 3rd gen Prius) - on 3rd gen (2010+) Priuses, use Eco mode - it reduces the threshold at which the ICE runs to provide cabin heat (when HVAC temp is set to high) from a coolant temp of 145 F to 114 F You won't need to the above once the ICE is warmed up enough, but it can cool down enough where it'll need to run to provide cabin heat. Since the Prius has no engine or coolant temperature gauge, there's no way to know how close it is to needing to run unless you have something like a ScanGaugeII : Linear Logic - Home of the ScanGauge or Torque — OBD2 Performance and Diagnostics for your Vehicle.