Q: I have a 2007 Toyota Prius and am very pleased with its performance. I do have a couple of concerns though. Some of my friends also drive a Prius and they seem to be getting 5 to 10 miles per gallon more than I do. I do not push the car hard, so I don't think it is my driving style. Things I have noticed are that in the winter I get much less performance than summer, as much as 10 mpg. Also, in the two times I have had the car serviced, I noticed significant reduction in gas mileage for several tanks of gas. I have read that the Prius' engine should not come on until the car is at 25 MPH. Even with minimal acceleration, I see it start and sometime stay on for several seconds before the electric motor cuts in. Is this normal? Can the Prius be tuned? - Marion M., Osseo
A:A number of variables affect gas mileage, including the way the car is used, state of tune, quality of fuel, speed at which it is operated, weight carried, aerodynamic and rolling resistance, etc. Manufacturers also make changes each model year, which can create subtle differences in weight, performance and fuel economy.
Looking specifically at your car and your friends' hybrids, here are some possible reasons for the differences. Contrary to standard gasoline automobiles, hybrids have inverted city and highway efficiency. While other cars do better on the highway, running at constant speed without stops and starts, hybrids tend to be more efficient at slow speeds when the electric engine is the predominant power source. Toyota's published stats for the Prius for 2007 and the several years before that were 60 mpg city, 51 mpg highway. That difference falls within the 5 to 10 mpgs you note compared to your friends. If your commuting and other driving has you on highways a lot more than your Prius-owning friends, that could contribute to your lower fuel economy.
For 2008 and 2009 models, Toyota adjusted its figures to 48 city, 45 highway. Given the same displacement and power ratings for those models and the slightly older ones, the adjustment is likely a straightforward correction from calculated mpgs to what owners were actually achieving. If your friends are generalizing about mileage based on expectations, rather than giving you the exact figures per tankful, that too could affect how well your car stacks up against theirs.
Tuning is also very important to fuel economy and yes, your Prius can be tuned (or out of tune). When an engine is badly out of tune, you can hear it and feel it - it doesn't run or idle smoothly; it is usually noisier and sounds strained. But subtle differences, like running too rich (too much fuel in the fuel/air mix of your gasoline engine), can waste fuel too, sending more into the engine than it can burn. Your service provider can monitor the exhaust gases to determine whether your gasoline engine is performing optimally.
Your car's getting poorer gas mileage after each servicing is puzzling. I have read about owners saying the dealership adjusted tire pressure (reducing it from what the owner was used to running) and that this reduced fuel economy. It could also be coincidence. Electric vehicles, including the electric motors in hybrids, are less efficient in cold weather. Because a hybrid's batteries are recharged by the gas engine, doing that work requires more gasoline energy during the winter than in warm months. If your servicings have been in the fall and coincided with colder temperatures, that would contribute something to the drop.
Economy also depends on proper coordination of the gas and electric engines. Your Prius' engine management controls will determine when the gas engine kicks on and off. If it's running more often than it needs too, your car will burn more gas. Your mechanic should be able to use manufacturer's performance specifications, along with any diagnostic error codes displayed, to determine whether the hybrid system is functioning properly.
