Toyota Prius Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
215 Posts
Discussion Starter · #1 ·
Could someone knowledgeable please explain something?

The Prius sticker claims roughly 50/freeway, 60/city.

At the same time, everyone seems to agree that short trips net the worst mileage.

If that's so, why is the city MPG so much higher. Aren't city trips almost ALWAYS going to be on the shorter side? (And, by the way, even when I spend a lot of time driving in the city, it seems all the stopping and accelerrating gives me much lower MPG's).

Does this make any sense, or am I nuts?
 

·
Registered
Joined
·
173 Posts
It depends on if you define short as miles or minutes. The first 5 or 10 minutes are less MPG because the car has to warm up.

Freeway MPG is usually less because of increased wind resistance.

In the Boston area, sometimes my longest trips (in minutes) are city driving and the shortest are on the interstate. I get better MPG in start and stop traffic with my Prius (although that is often technically on the freeway) than when I am at freeway speeds.

Here's how the sticker fuel economy values are obtained:

http://www.fueleconomy.gov/feg/how_tested.shtml
 

·
Registered
Joined
·
38 Posts
It has been my experience that the "optimal" driving road for the Prii are those with speed limits in the 30-40 mph range. I have a 20-mile commute to work that has multiple routes I can take. If I go the highway and stick to 60-65 mph (Speed Limit = 65 mph), I average in the low 50's for mpg. If I go a back way with speeds in the 30-40 range where I can take advantage of pulse-and-glide, I've been getting in the 60-65 mpg range (I'm not kidding!).
 

·
Registered
Joined
·
1,000 Posts
It's because the EPA's 'city' measurement is *NOT* a 5 minute trip here, a 10 minute trip there. It is more akin to a 'downtown drive'. It's a drive with a max speed of 35 mph, with lots of stop-and-go. (I can't seem to find the official explanation of what the EPA measures, my Google-fu is weak today.)

On a related note, yesterday I got 61 mpg over 28 miles of 'city' driving... (City highway, nominal speed limit 45, but with a few 25 construction zones, averaged 30. 12 miles out, 12 back, with an extra couple errand trips.)
 

·
Premium Member
Joined
·
888 Posts
Do not think of EPA numbers as a target of what you should be getting with the car, consider them a level barometer by which you can compare vehicles in the same class. Since the test done on each vehicle is highly standardized and follows a very exacting pattern of driving events, you end up with a very level indidator of MPG performance, however the methodology produces rather high numbers for most vehicles, higher than they will get in the real world.
 

·
Registered
Joined
·
1,233 Posts
I think it's ironic that I get the worst mileage downtown than I do in the suburbs. Mostly cause the crawling traffic drains the battery and doesn't give the motors a chance to regen via momentum or engine.
 

·
Registered
Joined
·
215 Posts
Discussion Starter · #7 ·
re: mileage

In that case, I think they should clarify their definition of "city" driving. In true city driving—even beyond the first 10 minutes—there are constant stops and starts. It seems impossible to approach the numbers previously mentioned. Suburban areas, with long stretches at 35 - 40mph, do not constitute city driving to me!
 

·
Registered
Joined
·
1,000 Posts
Yeah, I can get 100+ mpg for 10 minutes in true 'city' driving, but then I'm out of battery and I drop to about 35.

Here we go, my Google-fu is back.

EPA Reviews Fuel Efficiency Tests - American International Automobile Dealers.

According to Friedman’s group, some of the more glaringly outdated driving assumptions and conditions that are still being used by the EPA in its efficiency tests include:

* Low highway speeds. The EPA test assumes an average car speed of 48 mph and a maximum of 60 mph. Most state highway speed limits are now set at 65 mph or higher. At those speeds, fuel economy can drop by almost 10 percent to 17 percent, compared to 55 mph.

* Easy acceleration. The maximum acceleration rate is 3.3 mph per second, or equivalent to taking about 18 seconds to go from 0 to 60 mph. By the EPA’s own data, most drivers today accelerate nearly five times harder. The harder the acceleration, the more gas wasted.

* Overestimated trip lengths. The EPA’s "city" tests assume a trip of 7.5 miles. But even recent EPA figures show the average urban drive lasts five miles or even half that, at 2.5 miles. Shorter trips mean car engines do not have the time to warm up and operate efficiently.

* Exclusion of air conditioning and other accessories. Rare luxury items 30 years ago, air conditioners are practically standard features on nearly every new vehicle in the United States. Heavy use of air conditioning and other accessories mean a severe reduction in fuel efficiency -- especially in stop-and-go traffic conditions.
 

·
Registered
Joined
·
2 Posts
PRIUS fule economy AGAIN

Hello -

Trick question - how many pints of Gasoline are in a Standard EPA Gallon, used for testing emissions and estimating fuel economy?

CORRECT ANSWER - NONE! There are NO measurements of gasoline flow or consumption made during the EPA emissions or fuel economy test, believe it or not!

Gasoline consumption is an ESTIMATE by the EPA fabricated from exhaust gas by-products, which are sampled during the famous EPA emissions tests, and hence the term ESTIMATED City (or Highway) mileage.

However, the Engine does not burn all the fuel it receives; some fuel is converted into non-gas by-products called particulates. Hence, sampling only the gas by-products gives unrealistically high economy ESTIMATES (but we all have learned that)! The most notorious of these is particulate carbon.

The EPA must filter out the particulates (eg, carbon) from the exhaust gas stream - to prevent damage of their expensive gas sensors - and knowing they are 'missing' a by-product, they use a gasoline-like fuel that possesses a uniform length hydrocarbon chain to provide minimal (or approximately known) particulate carbon. Hence, the ESTIMATED fuel economy is based on exhaust gas-sensor readings and estimates of the particulate carbon present.

Unfortunately, you can not buy the uniform length hydrocarbon gasoline the EPA uses in their emissions test - your corner gas station sells a blend of liquid hydrocarbons that range in boiling point temperature from approximately 65F to 400F degrees...and the higher the boiling point, the more particulate carbon is produced. Ever wonder why OIL fires have such dense black smoke - that is due to presence of particulate carbon in the exhaust smoke. Diesel engines, of course, use FUEL OIL - and what is their smoke color?

Thus far we know: Carbon provides the majority of gasoline's combustion heat, and particulate carbon is un-oxidized (unburnt) gasoline that produces NO energy as it passes through your engine, and is unmeasured (only minimally estimated) during EPA testing. So, now you know about the EPA Fuel Economy testing? NOPE, more to come!

Lets talk about wind resistance or drag - the EPA emissions tests are performed on a STATIONARY dynometer, where the vehicle does not move and the body experiences no air drag. Hence, the EPA ESTIMATES can not take into account the shape of - say a Prius - or comparatively of a Pickup truck or van.

Hence, the EPA Higway Fuel Economy ESTIMATE is for an average speed of 49 MPH, where vehicle air drag has minimal effect on the (emissions or economy) result. Ever wonder why there was no TOURING speed (65+ MPH) economy estimate? They can't estimate that until EPA places their dynometers within a programmed wind tunnel to create air flow that matches the EPA emission test speeds (CITY AND HIGHWAY).

Now, why does the Prius have a higher City MPG ESTIMATE than Highway? because Prius shuts off the IC engine at stoplights and severe traffic, and traction batteries do not produce particulate carbon or other exhaust byproducts when they move the vehicle. The EPA must use the same corrections for the battery that they apply to idling IC engines, and hence their ESTIMATE overstates the result.

Because EPA do not account for air drag, their Highway Prius estimates might reasonably be expected to understate mileage. My own 2005 Prius delivered 53+ MPG (recent 270 mile trip from Glen Burnie Maryland to Pittsburgh PA) from pump gasoline, which is more than the EPA Highway estimate of 51 MPG. Expect more improvement as the engine accumulates more than the current 2,700 miles and 'breaks in' the planetary gears and the piston rings. The actual city mileage is about 46 MPG; when it was purchased it delivered between 43 and 45 MPG until 1,000 miles were reached whereupon things kept getting better.

When was the last time you owned a vehicle that BEAT the EPA numbers? Well, don't expect accurate EPA fuel ESTIMATES until EPA begin to accept particulate carbon as; a pollutant, and have to actually measure it.

WilliamD
 

·
Registered
Joined
·
7,161 Posts
I don't see how one could simulate wind drag in a stationary situation. Wind drag creates an engine load. How does one do that in a wind tunnel and a dynamometer?
 

·
Premium Member
Joined
·
2,815 Posts
The EPA definition of "city" and "highway" driving is based off of American's driving patterns in the 1970s. In the 1980s, they applied a downward multiplier to closer meet the 1980s' driver, but the tests haven't been revised. The tests are only good for comparing one car against another (since all cars undergo the same test), but do not match the modern Americans' driving style so cannot be used for prediction of your MPG.

Photos and description of the tests (done in a lab): http://www.fueleconomy.gov/feg/how_tested.shtml

http://www.fueleconomy.gov/feg/info.shtml#estimates
How are fuel economy estimates obtained?

The fuel economy estimates are based on results of tests required by the U.S. Environmental Protection Agency (EPA). These tests are used to certify that vehicles meet the Federal emissions and fuel economy standards. Manufacturers test pre-production prototypes of the new vehicle models and submit the test results to EPA. EPA re-tests about 10% of vehicle models to confirm manufacturer's results. The vehicles are driven by a professional driver under controlled laboratory conditions, on an instrument similar to a treadmill. These procedures ensure that each vehicle is tested under identical conditions; therefore, the results can be compared with confidence.

There are two different fuel economy estimates for each vehicle in the Fuel Economy Guide, one for city driving and one for highway driving. To generate these two estimates, separate tests are used to represent typical everyday driving in a city and in a rural setting. Two kinds of engine starts are used: the cold start, which is similar to starting a car in the morning after it has been parked all night; and the hot start, similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.

The test used to determine the city fuel economy estimate simulates an 11-mile, stop-and-go trip with an average speed of 20 miles per hour (mph). The trip takes 31 minutes and has 23 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. The maximum speed is 56 mph. The engine is initially started after being parked overnight. Vehicles are tested at 68 F to 86 F ambient temperature.

The test to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile trip and averages 48 mph. The maximum speed is 60 mph. The test is run with the engine warmed up and has little idling time and no stops (except at the end of the test).

NOTE: To make the numbers in the Fuel Economy Guide more useful for consumers, EPA adjusts these laboratory test results to account for the difference between controlled laboratory conditions and actual driving on the road. The laboratory fuel economy results are adjusted downward to arrive at the estimates in the Fuel Economy Guide and on the labels seen on new cars, light trucks, and vans. The city estimate is lowered by 10% and the highway estimate by 22% from the laboratory test results. Experience has proven that these adjustments make the mileage estimates in the Fuel Economy Guide correspond more closely to the actual fuel economy realized by the average driver.


Published information from Toyota:

Tips for better Prius Gas mileage (near end of Spring 2004 newsletter):
http://www.toyota.com/html/hybridsynerg ... ng2004.pdf

Prius Fuel Economy: Explaining the EPA Ratings
Toyota explains what the EPA ratings actually mean, and lists ways to
improve your MPG
http://autos.groups.yahoo.com/group/toy ... sage/71431
http://autos.groups.yahoo.com/group/Pri ... ssage/2742
http://www.priusonline.com/viewtopic.php?t=1079

Prius Fuel Economy Factsheet:
http://www.toyota.com/images/vehicles/p ... conomy.pdf

Can I expect to get over 50MPG in the new Prius?
(see question #6): http://www.toyota.com/vehicles/2005/prius/faq.html


meanwhile, on the topic of published EPA ratings, I suggest reading
the following article (with plenty of citations for source data):
http://www.bluewaternetwork.org/reports ... ehood2.pdf
"FUEL ECONOMY FALSEHOODS: How government misrepresentation of fuel
economy hinders efforts to reduce global warming and US dependence on
foreign oil" by the Bluewater Network, 2002
 

·
Registered
Joined
·
2 Posts
DanMan32 said:
I don't see how one could simulate wind drag in a stationary situation. Wind drag creates an engine load. How does one do that in a wind tunnel and a dynamometer?
The usual Wind Tunnel Drag measurements involve placing steel wheels on the test subject (to minimize rolling resistance and static friction) and then onto steel rails in the wind tunnel. A tether is fitted to the test subject (to prevent it rolling when the wind starts) and then a scale is connected (to the tether) to measure and record the drag-force while the wind is gradually increased.

This kind of thing is also used with ship-models to determine hull-flow-drag (except the steel sheels are not needed if the model floats)...

It is a simple thing to multiply the drag-force (in lbs) by the wind speed (in ft/sec) to obtain the power required, then divide by 550 to obtain the horsepower (one HP is 550 ft-lbs/sec) required to overcome drag at the wind (vehicle) speed. There are also 747 watts per horsepower (if I remember correctly), a useful fact for a chassis synometer...

The EPA mileage test would then be run on a static dynometer, except the engine load (in watts) would be added to match wind resistance HP (proportionate to the speed of the vehicle drive wheels, sensed by the Dyno rollers)

Some years ago, the auto-makers were objecting the EPA was not giving them mileage credit for their newer, stream-lined bodies (they had integrated bumbers and drip rails, changed hood lines, roof lines, and windshields, etc)

Now you know why, and how, the EPA could not credit them correctly - not that it matters, because effects of stream-lining are pretty well lost (except possibly on Hybrids) in the noise of no particulate carbon data...there is so little actual measurement in the EPA mileage estimates that is absolute reason enough to call then ESTIMATES!

NOTE: Prius has a very nice drag coefficient - nearly matches a Formula race car, because race cars must have Air Intakes (in the cool air stream) to feed 600+ HP engines. That limits their drag efficiency.

If you want to make the EPA mileage estimates better, demand particulate carbon become a named pollutant.
If you wish to make Automobiles better, demand particulate carbon become a named pollutant AND that both the air and tire (rolling) resistances become an integral part of the EPA miltage estimates.

Any bets the US Auto Lobby will set a new standard of resistance at the prospect of those changes? Pun intended ;)

WilliamD
 

·
Registered
Joined
·
2,285 Posts
"The maximum acceleration rate is 3.3 mph per second, or equivalent to taking about 18 seconds to go from 0 to 60 mph. By the EPA’s own data, most drivers today accelerate nearly five times harder."

18 seconds divided by 5 is 3.6 seconds.

How many cars can go from 0-60 in 3.6 seconds?
 
1 - 16 of 16 Posts
Top