Winter's Effect on the Fuel Efficiency of the Toyota Prius, Ford C-MAX, and Other HybridsIf you live in a part of the country (or world) where it gets cold in the winter, you’re likely to have experienced a drop in your vehicle’s fuel efficiency. The colder it gets, the harder your car or truck works to get up to an optimal operating temperature, and the more gasoline it consumes in the process. This effect is even more pronounced in hybrid vehicles due to the nature of their drive trains. The drop in cold weather mileage can be substantial.

There are a number of reasons why cars get poor gas mileage in colder temperatures:

  • Engines are significantly less efficient when cold. Oil thickens at lower temperatures, increasing friction.
  • Increased idle time, whether at morning warm-up or while sitting in parking lots (in order to keep the cabin heat flowing), wastes gasoline.
  • Gasoline is a cocktail that changes depending on the season. Winter gasoline formulas differ from summer formulas, in that they may contain butane and other chemicals that can effect fuel efficiency.

Popular Mechanics has an excellent article on the differences between summer and winter gasoline. Blame it on the RVP…

The different grades of gas are measured on a system of RVP, or Reid Vapor Pressure, which is measured in pounds per square inch (PSI). The higher the RVP number of a particular gas blend, the easier it is to vaporize and the worse it is for the environment. All gasoline blends have to be below 14.7 PSI, which is normal average atmospheric pressure. Any number higher than that and gasoline would become a gas.

The summer-to-winter MPG difference is even more pronounced with the hybrids.

I recently spent two weeks testing a 2013 Ford C-MAX Hybrid and witnessed the climate-driven drop in fuel economy, first hand. I documented individual commuting segments, taking note of the effects of outside temperatures on mileage. The colder it got, and the shorter the commute distance, the tougher it was to get close to the official specs. (While I have not personally had the opportunity to test the Toyota Prius for winter fuel economy, you’ll find a good number of discussions in the forums about the winter MPG drop.)

So what’s the cause of the drop?

Hybrid vehicles need to fully warm up in order for their electric drive trains to function optimally. Blame it on safety. With advanced hybrid systems, the gasoline engine needs to start up instantly. If the engine is cold, that could present an issue, so the software prevents the system from going into full hybrid mode until the engine is properly warmed up. Better safe than sorry.

The cold weather’s effect is most pronounced in hybrid vehicles that are used for relatively short commuting distances. With a short commute, the engine may not reach operating temperature (and the hybrid system may not be fully functional) until later in the abbreviated driving cycle. With a longer commute, the engine warms up earlier in the cycle. If a warm up takes five minutes and your drive to work is 10 minutes, your hybrid drive train will only be operating at peak efficiency for 50 percent of that time. If your drive to work is 25 minutes, your drive train will be fully functional for 80 percent of that time. I’m not advocating taking the long way to work, but it’s a numbers game.

There are two basic ways to speed up the warmup. If you can keep your hybrid in a garage overnight — tucked away from the cold — it will take less time to warm up to operating temperature. If you have access to an electrical outlet and can have an optional engine block heater installed, you’ll be in even better shape.

I’m not an advocate of either long warm ups or simply slamming the car into drive seconds after the key’s been turned (or the start button’s been pushed, as the case may be). I prefer to let the vehicle have a short warmup to get the juices flowing, so to speak, before taking off. Start the car, put on your seatbelt, adjust the mirror, turn on your tunes, and you’re good to go in half a minute or so. If it’s so cold that you need to deice the windows, that’s a whole ‘nother issue (and all the more reason to keep the car in a garage, if possible). Needless to say, it’s a blessing to have heated seats in the wintertime.

A number of Toyota Prius owners have taken to using grille blockers in colder weather as a way to reduce the amount of freezing air that enters the engine compartment. This practice has been going on for years with cutting-edge Prius owners and it appears that it may be catching on with some new C-MAX owners, as well.

Image: Frozen Car shared by dmvyanks (via Flickr)