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Why Don't I get the EPA Mileage?? (Written by efusco)

Posted on: Monday 03rd of April 2006 01:45:05 PM


Particularly for those just getting their Prius in the fall or winter months this must be one of the first questions and most common questions I've seen posted. I think I posted something similar myself when I first got my Prius in late Oct. of 2003.

Toyota, rightly, advertises the Prius with the EPA determined gas mileage of 51 highway, 55 combined, and 60 city. I think that as part of the excitement of getting the car we all anticipate seeing those numbers 'right out of the box' when we pick up our car. More often than not that doesn't happen. In this article I'm going to try to clarify the reasons why. I'll also offer some suggestions on how mileage can be improved toward the end.

When the EPA tests cars and rates their gas mileage they don't actually drive on a road. They use a device called a dynometer. They use the exact same pattern of driving that's been used for every tested vehicle for over 20 years. The "city" portion is nothing like what most of us consider city driving...there are many stretches of 1/2-2 miles at 21mph. And although there are many stops built in there is nothing like the stop and go driving of a real city. Thus, many people in 'real' cities don't see anything like the city mpg numbers.

The highway test portion is equally outdated with an average speed of 48mph. For those of us who use the interstate transit system in the US and know that posted limits of 70mph are, in reality, minimum speeds with most vehicles in the 75-80mph range this portion of the EPA test is equally unrealistic.

To further exacerbate the issue of inaccuracy with 'real world driving' the EPA tests all the cars in a temperature and climate controlled building at 68-86 degrees Fahrenheit with a vehicle that is already warmed up (highway only, city portion is cold start) and A/C turned off. It assumes a flat level road (i.e. no hills). It measures exhaust CO2 to determine how much fuel was used.

See How Vehicles Are Tested in detail at this link, complete with photos.

a)Hybrids (as other cars) have their lowest MPG in the first 5-10 minutes of driving while the vehicle warms up. Thus, for those of us with only a 5-10 mile commute we're unlikely to ever get the car into it's most efficient operation condition.

b)Due to the battery power and the range of SOC of the battery, particularly on the city portion of the test, much of the driving may be under battery power and a small portion of energy used is never measured by the EPA (as they have no means to do so) thus giving a slight higher MPG rating than is really possible. They say that the battery SOC must be at the same level at the beginning and end, but even so there is some inaccuracy in that readout.

c)All cars perform less efficiently and require longer warm up times in colder weather...even at 50 degrees there's an impact. This becomes most noticeable in a high mpg vehicle like the Prius.

The best way to think about this is to consider the Prius compared to another vehicle. If the Prius gets 55mpg in 72 degree weather but suffers a 10% decrease in mileage at 45 degrees that means that your gas mileage will drop to 49.5mpg--you're getting 5.5mpg LESS than you were at 72 degrees!! Pretty demoralizing. If you're driving a Camry that gets 25mpg at 72 degrees and suffer that same 10% hit at 45 degrees you drop only to 22.5mpg...hardly even noticeable at a fill up since it's only 2.5mpg less than before. It's the same 10% but it's relatively more due to the lower 'gallonage' of the Prius (that is uses less gas for distance traveled).

Especially early in the Prius ownership there are a number of other factors that come into play. Many of which will work themselves out over time.

1)Break in--almost universally people notice an improvement in mileage at about the 5000-6000 mile mark. The first generation Prius owners and a few 2nd generation Prius owners who've passed the 20k mile mark also note a 'secondary break-in' period. It seems that the engine friction, wheel bearings and other moving parts 'loosen up' over that first 5k miles. Also, the High Voltage Nickel Metal Hydride (NiMH or HV) battery becomes more efficient over time.

2)Technique/style/learning curve--I firmly believe that the Prius driver intuitively or through trial and error improves his/her driving technique over time better take advantage of the most efficient modes of driving their car. Not all drivers do this, but it becomes almost a game. With time one learns to accelerate the vehicle in the most efficient manner. He/she learns to anticipate stops and begin coasting or gliding to those stops without wasting power to get there or putting undue stress on the friction brakes.

3)Warmer temperatures--all praise to summer. The Prius is at it's most efficient in warm weather...particularly in that range of temperatures where A/C use is not necessary but where air temperatures are in a range that is 'comfortable' to the car and HV battery. Almost everyone sees a dramatic improvement in fuel economy in those summer months and reports of exceeding the EPA numbers begin to frequently appear on the forums!

4)A/C use--there is a significant impact on mileage with A/C use. Unfortunately Toyota did not make it easy or intuitive to learn how to get to the 'vent' mode to allow circulation of outside air without engaging the actual heating/cooling system. In cold weather I either don't use the AC at all since my car stays in the garage at home and usually remains warm enough (with my coat on) that I don't have to use it, or I turn it on 'Auto A/C' at 65 degrees--the lowest setting. That is almost always comfortable when I'm wearing a decent winter coat. If my wife/family are with me I do need to increase that, but I still try to keep it below 70 degrees. Likewise, in the summer, I either just use the 'vent' setting or set the temperature to something b/w 78-81 degrees (depends on how much sunlight is shining directly into the vehicle as to what is comfortable enough).

5)Weight in vehicle-- Although most small articles won't make a major impact you don't wanna be carrying a bunch of unnecessary stuff in the car all the time. If you always have the car full with the spouse and 3 kids and all their necessities of life in the hatch it will impact your mileage.

6)Tire and road rolling resistance--The tire rolling resistance of the OEM Goodyear Integrity tires is very low. If under inflated, however, that resistance increases creating a significant impact on gas mileage. Many of us have decided to use higher tire pressures than Toyota recommends to further improve fuel economy by a small amount and to improve tire wear, handling, and road hazard safety. Some argue that this causes an uncomfortably rough ride and might be a safety issue. Whatever pressure you choose to use always be sure to keep the 2psi higher pressure in the front tires over the rear and to stay within the tire manufacture's maximum rating (44psi for the Integrity).

Road rolling resistance is out of your control...but is a factor none-the-less. If you have rough roads, wet roads, gravel, etc. that increases rolling resistance and reduces mileage. Ideally dry smooth roads are best.

7)Other weather related issues--Wind speed and direction have an impact, as does rain (see above), humidity, elevation and barometric pressure. I won't linger on those issues, but they do factor in.

8)Speed, braking, acceleration, and anticipatory driving--The MFD's Energy and Consumption screens present a degree of feedback unprecedented in any other stock vehicle. It acutely increases our awareness of what is happening from moment to moment, every 5 minutes, and over time.
This has affected the way I drive dramatically and many other Prius drivers notice the same. I accelerate fairly briskly getting to speed. I don't floor it, but maybe depress the pedal 1/3rd way or so. Once I get over the 25-30mph mark I begin trying to 'Dead-band'--on the energy screen all power is from Engine (ICE) to wheels with no arrows to or from the battery. This is the most efficient power phase as there are no losses from converting the energy to potential energy in the battery back to kinetic energy for the wheels.
I also try to anticipate stops well ahead of time. I've learned the timing of the lights on my usual commute and can often see them far ahead and know if I can make it by maintaining my current speed or if it will change before I get there. I watch the other traffic as well trying to anticipate what they might do I think that kind of anticipation makes for a safer driver in addition to making it possible to drive more fuel efficiently. When I know there is a stop say 1/2 mile ahead but there is traffic behind me I'll go into a "glide" mode. I take my foot completely off the accelerator for a second (you'll see just green arrows on the energy screen) then I very lightly depress it again to a 'no arrows' condition on the energy screen. That eliminates the 'drag' of the regenerative braking that occurs with the foot completely off the pedal. If I need to maintain a little more speed I'll press just enough to get the yellow arrows of electric only power. Once I see that I must stop then I'll firmly, but not hard, brake to try to use just regenerative braking (no friction brakes). When possible I try to avoid coming to a complete stop with this anticipatory method. If I can coast up to a red light and still be traveling 10mph that is 10mph plus no static friction from a stop that I have to use energy to get back.

Also, I almost never exceed the speed limit any more. The faster you go the lower your fuel economy will be. Esp. at speeds over 60mph the impact of air resistance, even in the low Cd Prius, is huge. Maintaining a steady speed that doesn't impede traffic allows the Prius ECUs to find the most efficient combination of ICE and Electric motor use.

As always, however, PLEASE make safety your first priority. When you can safely take advantage of this type of anticipatory driving it is just a bonus.

9)Fuel Type--Summer gas has a higher BTU rating than winter gas due to pollution control issues in most parts of the US. Thus, you get more energy per unit of gasoline with Summer gas. Thus, in addition to the cold weather hit alone, now you get less 'bang-for-the-buck' from your gasoline. Make it another 5% and you're down into the mid-40s mpg area down from the 55mpg you may have gotten in the summer!! And it's completely out of your control.

Also, many parts of the country are beginning to use E10 gas (10% ethanol). This has an even lower BTU rating than winter gas (though has environmental benefits in much lower sulfur content) and thus lower gas mileage will be seen. Many people aren't even aware that they're using E10 fuel...the pump should have a small sticker notifying you of the fact.

10)Synthetic Oil--Some have stated they noticed a measurable improvement in MPG just by switching to synthetic motor oil. I'm not sure that this is true, but the synthetics do tend to have a slightly lower viscosity than mineral oils so it's possible it has some effect. That effect may be particularly noticeable in COLD weather where synthetic is clearly a superior performer.
I, personally, chose to use synthetic after extensive research based primarily on the slightly lower environmental impact of synthetic oil. Also factored in was the potential for slightly higher gas mileage and longer engine life. That has to be weighed against the significantly higher cost for synthetic oil which certainly outweighs any fuel efficiency improvements alone.

11)Short Trips--You'll almost immediately notice that your first 5 minute bar on the consumption screen will be in the range of 25-35mpg most of the time. Extra fuel is used to warm up the engine and catalytic converter in the first 5-10 minutes of driving. Thus, short trips are a mileage killer. If you must take short trips try to combine them so that there is minimal time for the ICE and CAT to cool down, thus maintaining the warmth longer. The 'coolant' stored in the 'thermos' will also be hotter to aid that process. Longer drives (over 15-20 min.) at moderate speeds (40-45mph) will show some of your best gas mileage...often in the 60mpg range.

12)Terrain--Those of you lucky enough to live in flat areas like New Orleans or Florida have a distinct advantage as well. The ECU is able t maintain a steady speed over the entire distance traveled...The Cruise control is truly your friend. Those of us who live in hilly or mountainous terrain take a pretty good hit on the steeper hills. Although one can sometimes recoup a little of the inefficiency on the down-hill portion, it's never as much as was expended getting up the hill in the first place.

13)Alignment--For persistent issues with mileage not meeting your expectations after factoring in ALL the items above, consider having your alignment checked and fixed. This is pretty critical to long term mileage and not always something that can be appreciated by 'feel' alone.

Multiple factors conspire to impact MPG and the relative impact seems greater in a higher mileage vehicle like the Prius though, percentage wise, it is probably not any worse than a conventional vehicle.

-Minimize AC use
-Maximize Anticipatory driving
-Consider higher than Toyota recommended tire pressures (many use 42f/40r, 40/38, or 38/36)

And finally: Be'll get better, there's nothing wrong with your car. We've just all been set up with some relatively unrealistic expectations and it's natural to be a little disappointed when those expectations aren't immediately met.

Another Great Resouce is

A thread at the Yahoo site:

See also the Official Toyota PDF on Fuel Economy
Direct Link to PDF on PriusChat

Also this is a very thorough article from Chevron on Fuel Economy that is well worth downloading and reading.

Evan E. Fusco, MD




Posted: Friday 23rd of February 2007 12:17:18 PM
I can't figure out how to edit this article, but a critical update needs to be referenced as of the 2008 model year, but also applicable to all other years.

The EPA FINALLY realized, as I stated above, that their testing methods were unrealistic. They've since added 3 new tests that are used to get more realistic numbers. These include a High Speed test that uses very rapid acceleration and decelerations with speeds up to 80mph. An AC section where the AC is used in lab temps of 95 degrees F. And cold weather testing at 20 degrees F with a cold start.

New Prius EPA ratings are
City-48 Hwy-45 Combined-46

Old numbers were:
City-60 Hwy-51 Combined-55

While the new numbers certainly look low I'm quite optimistic that the vast majority of Prius drivers will exceed the new EPA FE ratings by 25% or better...that's mostly b/c the new tests are also a bit unrealistic compared to 'every day' driving most people do. But at least there will be fewer people disappointed with their FE.

Evan E. Fusco, MD

Original Article on at:


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