How will temperature affect MPGe?

Ben Bowlin

The all-electric Nissan Leaf is displayed during the opening ceremony of the company's new headquarters in Yokohama, Japan, on Sunday, Aug. 2, 2009.
The all-electric Nissan Leaf is displayed during the opening ceremony of the company's new headquarters in Yokohama, Japan, on Sunday, Aug. 2, 2009.
AP Photo/Itsuo Inouye

Over the past week Scott and I have been looking closely at the EPA's newest measurement of fuel efficiency: the miles per gallon equivalent, or MPGe. MPGe was created to give car-buyers a better sense of how hybrid or electric cars perform in comparison to conventional vehicles. (For a closer look at the nuts and bolts behind this comparison, take a look at Scott's earlier post.) Just like MPG estimates, the numbers of MPGe estimates can vary, and it's good to know why.

We already know that a vehicle's MPG can be affected by numerous external factors. That's why you'll see two MPG ratings on most car stickers -- one for highway driving, and another for street cruising. Like the conventional MPG, MPGe can be affected by a number of external factors. Temperature is one of the most influential here. As every driver in colder regions of the world knows, a low temperature can make it harder to start your car -- but why?

Let's summarize the link above. If you've got a gas-powered car, you can run afoul of several low-temperature obstacles. First, gasoline evaporates less in low temperatures. Second, engine oil thickens in low temperatures. Third, batteries can have a tougher time in cold weather. Low temperatures slow down the chemical reactions that power batteries. When you're starting a car in 0 degrees Fahrenheit, your car's battery has less than half of its starting potential (even if it's fully charged).

This factor becomes even more important when we're talking hybrid or electric cars. Hybrid vehicles use their electric motors for low-speed driving, and the gasoline engine kicks in when there's a demand for more speed or power. In a cold environment, the gas engine runs less efficiently, essentially doing more work for less power. Running the heater can reduce the vehicle's efficiency. Additionally, the added possibility of ice, snow and low tire pressure can all adversely affect eco-friendly mileage.

So, while full-electric hybrid engine systems will likely experience lower efficiency in extremely cold weather, conventional engines are also affected. Bottom line: Extremely cold weather is going to impact the performance of all three engine types.

There is, however, some good news: If you drive a hybrid or full-electric vehicle, then your battery pack is less prone to freezing. Most manufacturers' batteries will perform at temperatures as low as -22 degrees Fahrenheit.

Do you own a full-electric or hybrid car? How has it performed in cold weather? Let us know on our Facebook page, or our Twitter feed. Heck, hit us up on either one.