DETROIT -- Many drivers of gasoline-electric hybrids never see their real-world fuel economy match the numbers on the EPA window sticker.
Because of hybrids' complex powertrains and other factors, the gap between real-world fuel economy and the window label is often greater than it is with vehicles powered by conventional gasoline or diesel engines.
And that discrepancy has led to lawsuits against Toyota, Honda and Ford by angry customers who expected to get the same or close to the fuel economy listed on the sticker.
In its test of hybrids, Consumer Reports cited the Ford C-Max Hybrid and Toyota Prius as two vehicles with the largest gap between the window sticker and the real world.
Here are a few factors that can cause these gaps.
- Temperature: Drivers who live in states where temperatures dive into the 30s and below see the biggest variances in fuel economy. It starts with the fuel. Winter-blend gasoline contains additives that lower the fuel's freezing point and allow for faster engine warm-ups. Winter-blend gasoline contains less energy than summer blends, causing fuel economy to drop.
The lubricants in the vehicle -- engine oil, transmission fluid and axle oil -- thicken in cold weather and require more energy to pump until the vehicle warms up and the lubricants thin out. Batteries and electric motors are also less efficient in cold weather. Low temperatures reduce driving range until the batteries reach about 70 degrees.
And drivers in cold weather tend to use more energy-gobbling accessories such as seat heaters, window defrosters and heated outside mirrors. All the vehicle's mechanical systems work harder and use more energy in winter.
- Vehicle condition: New vehicles with few miles use more energy until the moving parts have a small amount of wear. After about 5,000 miles, vehicles begin to deliver better fuel economy.
- Top speed: The amount of energy needed to propel a vehicle dramatically increases above 55 mph. According to Ford, fuel economy can improve 25 percent on some vehicles by reducing speed from 70 mph to 55 mph. Most hybrids tend to get better fuel economy in stop-and-go city driving, in which the electric motor does much of the work. On the highway, the amount of power sent to the wheels from the electric motor is greatly reduced on most vehicles, which means the gasoline engine is hauling around a heavy battery pack and other equipment.
- Driving style: Many hybrid buyers -- including those of Ford's C-Max Hybrid -- report higher mpg in real-world driving than what's on the EPA window sticker. Some drivers, called hypermilers, buy hybrids and drive with the intent of gaining maximum efficiency from the drivetrain. To achieve EPA-beating fuel economy, many hybrids must be driven at speeds at or below posted limits, especially on the highway, with the air conditioner off and with careful attention to acceleration and braking.