How to Get Better Fuel Economy From Your Prius
The Ugly Prius
Better Fuel Economy for the Everyday Prius Driver
This article is dedicated not to the hard core hypermiler, but rather to the everyday Prius driver that wants better fuel economy, or higher mpg, from their Toyota Prius. The Prius is currently the premier high mpg car in the US, but even so that already high mpg can be raised by understanding a few basic tips. Better fuel economy does not come from magical water injectors or from perpetual motion devices, but from careful driving and a good understanding of the operating principles of your car. The Toyota Prius hybrid car is different enough from normal cars of the past that it can pay off handsomely to do a little research on some of these differences.
Basic Differences With the Prius
Obviously, the Prius is an electric hybrid car—I'm sure you knew that if you already own one. What you may not realize are the economies that are possible as a result of those differences; just how to properly use those differences to achieve higher mpg figures takes a bit of study.
The Prius contains a high voltage, high capacity battery capable of propelling the vehicle to slow speeds (42 mph) without the use of the gasoline engine by using a pair of electric motors. Please note the nomenclature; "engine" refers to the gasoline engine while "motor" refers to the electric motors. I will be using these terms throughout the tutorial.
The engine in the Prius is different than ordinary car engines in that it is designed around the Atkinson Cycle rather than the normal Otto Cycle engine used in other automobiles. It is designed and tuned for efficiency and better fuel economy rather than power and has different operating characteristics.
The brakes on the Prius normally operate via regeneration - a term used to indicate that the energy of motion is being converted to battery power as the car is slowed. Actual brake pads, similar to other cars, are also used but only in very hard stops and below 7 mph where regeneration is not practical.
The transmission in the Prius is known as a CVT (continuously variable transmission) and has no "gears" as we think of them. The only shifting is between drive, neutral and reverse and in neutral and reverse the engine is shut down and motion is accomplished by the electric motors and/or momentum.
Finally, the Prius is designed for two things; better fuel economy and a minimum of pollutants. These two items are not mutually incompatible, but they do affect each other and those effects need to be taken into consideration.
These differences need to be taken into account when considering just how to get better fuel economy, and require different discussion at highway and urban speeds.
Prius Fuel Economy for the Highway
Although the Prius is classified as a mid-sized vehicle it is designed primarily for the urban environment. Nevertheless there are a few actions you can take to get better fuel economy when highway driving.
First on the list, of course, is to slow down. While the Prius has a lower air resistance than nearly any other car, it is still applicable and fuel economy goes down rapidly at higher speeds. I seldom drive over 70 mph and never over 80. For a 200 mile trip the difference in time required is only 30 minutes when going from 80 mph to only 65 mph and the difference in average mpg is quite large. Slow down and enjoy the drive more.
Maintain a constant speed, especially on flat ground. Changes in speed cost fuel and that fuel usage cannot be overcome. Cruise control is a great option - use it whenever possible.
Much of my own driving is through mountainous areas, however, and I find that a little better fuel economy can be found by accelerating slightly uphill. This flies in the face of common sense, but there is a reason for that in the Prius. The Atkinson engine is carefully tuned to operate best at a particular engine speed, about 2100 RPM. On most hills, this engine speed will not be reached and the engine will not, therefore, operate at peak efficiency. Without a tachometer, you cannot tell what the engine speed is, but a good rule of thumb is to keep the instantaneous mpg figure on the energy screen to about 1/2 of the speed. In any case you should pick a top speed that you will attain and try to increase speed slowly so that speed is found at the top of the hill. On the way down, speed should be allowed to decrease slowly until the cruise control again takes over. Do not allow the car to coast on gentle hills (green arrow on the MFD), but keep the engine running just enough to slowly shed speed. You may find on some hills that the engine will shut down and run on battery only. Practice will tell you what throttle setting is needed for different grades.
I find that I average around 45 - 50 mpg on the freeway. A little disappointing to me, but I drive in mountainous areas and that always costs extra fuel. In addition it usually windy and that also hurts. I normally drive at 70 mph and I realize that some savings could be saved by slowing down, but that is the choice I make. I usually have either the heat or AC on and that also costs, but is again a personal choice. My comfort is more important than saving 50 cents on my trip.
MFD Screens Relating to Fuel EconomyClick thumbnail to view full-size
Fuel Economy in Urban Areas
This is where the Prius shines, and by far where the better fuel economy is achieved. I always reset the mpg counter when I fuel up and while I have achieved as high as 65 mpg for a tank, my normal average is around 53 mpg. It also includes driving done by my wife, who makes no effort to increase her economy and always hurts my average.
Tip #1 - Do not drive on battery alone! I realize that this is again counter intuitive, but there is a good reason - 100% of the battery charge you are using by driving on battery alone is achieved by the use of gasoline, and there are always energy losses in every system. Even regeneration by braking requires gas to get to the speed you are slowing down from. I make one exception to this rule; the last 3 or 4 blocks into my home. By using the battery here I leave the battery in a discharged state, but it is a state to be recovered from the next morning. You see, the Prius computer will run the engine on start up to warm the emission system to its operating temperature. You have probably noted that after a few seconds you cannot drive on battery in these conditions; it is because the computer is requiring the engine to run whether it actually needs to or not. There are other exceptions, of course; to impress your new passengers (always fun!) or to move quietly from a garage or parking area. Such things won't cost you much, just don't do it as a matter of course because you think it saves fuel. It doesn't.
Instead, drive with the engine off without using the battery. Watch the MFD; the blank screen with no arrows is the one you want. The preferred method is to speed up to a little more than is actually desired, then let off the throttle. This will produce regeneration, or the green arrows; reapply throttle slowly until the green arrow disappears but the before yellow arrow (electric motor) comes into play. The car will gradually slow down; I let it happen until it reaches about 5 mph below what I really want, then repeat the process. Progress down the road is a constant variation in speed, from 5 mph under to 5 mph over my desired speed. During acceleration the instantaneous mpg is again held to about 1/2 of the speedometer reading, just as it is done in climbing hills, as that gives the best approximation of the ideal speed of the gasoline engine.
Tip #2 - Maintain your speed. No, I'm not talking in circles; what I mean by that is don't slow down or stop any more than necessary. Try to time stop lights so that you don't stop; slow down (using the "no arrows method) ahead of time so that you drive through them as soon as they turn green without stopping. If a car in front you slows to turn, slow down earlier in order that you maintain as much speed as possible and drive by the other car at as high a speed as possible.
Tip #3 - Try to plan your trips so as to not make any more stops than necessary. Even warm, the computer will run the engine unnecessarily each time it starts. I go to yard sales a lot; if possible I will leave the car on at each stop. The engine doesn't run while stopped and there is no "startup" cylce to worry about.
Tip #4 - If you live in a cold climate, consider purchasing a block heater to help keep the engine warmer; it will come to operating temperature quicker each morning.
Finally, whether you are driving in the city or country, keeping your Prius in good mechanical condition will pay dividends in fuel mileage. On thing that most people ignore is the small 12 volt battery, but as that battery ages and becomes incapable of holding a charge it will affect your fuel mileage. If necessary change the battery out for a new one before it begins to go dead on a weekly basis.
Try these tips in your own Prius; I assure you that they will work and will increase your fuel economy. I also find it fun to the get the most I can out of my car - to use it more nearly to its maximum potential.
Questions & Answers
I have a 2009 Prius with 108,000 miles and have been only averaging about 36-37 MPG over the last year. Even after an oil change, I have not seen any appreciable changes. I live in South Florida and run my A.C. nearly year round, but even still, this seems like an exceptionally low MPG. Any suggestions?
Yes, AC will most definitely affect your mileage, but just as you say it shouldn't be that much. You didn't say, but I assume it used to be higher.
Have you changed your little 12V battery? They slowly go bad, just like all batteries, with the result that the Prius tries, hard, to charge it up all the time. That might be something to consider - it is used primarily to click on the big relays to the high voltage battery, but that's about all so it isn't like the battery in other cars that you know is going bad because the starter will barely turn over. I know I saw the same big mileage drop at about the same time you are and replacing that small battery took care of most of it.
Again, you didn't say, but new tires will also hurt although to do so for a full year seems unlikely.
The only other thing is to take it to a shop for a complete check-over. A tune up on the engine (new plugs, air cleaner, etc.) and to look for anything else.
I was wondering whether you had any experience in using the B-mode in a more recent Prius. I just got a 2018 Prius Eco, and I´m not sure under what circumstances the B-mode helps to conserve fuel. Is it only on extended downhill sections, or can it be used as a regular setting for the city or highway?
I do not. But you should consider that any energy coming out of that battery will be replaced by burning fuel unless it is a plug-in hybrid. Given that, I don't see a battery only mode saving much at all. If you're changing parking spots, or in a slow line getting out of a parking lot, it might help conserve a little.
© 2010 Dan Harmon