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Under New EPA MPG Method, All Estimates Drop; Hybrids Retain Most of Their Advantage Relative to Conventional

26 February 2007

Epa_mpg
Hybrids retain most of their fuel economy advantages over their conventional counterparts under the new EPA method for estimating mpg, despite a decrease in the estimate resulting from the new method. Click to enlarge.

The EPA has published estimates of the affect of the revised methods for estimating vehicle fuel economy—which apply to model year 2008 and later vehicles—on earlier model year cars as a way to educate consumers on the likely impact of the changes.

Compared to today’s estimates, the city mpg estimates for the manufacturers of most vehicles will drop by about 12% on average, and by as much as 30% for some vehicles. The highway mpg estimates will drop on average by about 8%, and by as much as 25% for some vehicles. (Earlier post.)

Although the fuel economy estimates for hybrids drop along will those for all vehicles, hybrids appear to retain the majority of their fuel economy advantage compared to their conventional counterparts.

While the combined mpg estimate for the 2007 Camry hybrid, for example, drops 13% from 39 mpg under the old method to 34 mpg under the new, the hybrid’s efficiency advantage compared to its conventional cousin only decreases 3 percentage points from 39% to 36%.

The 2007 Prius, with no conventional model for comparison, would take a 16% hit in combined fuel economy under the new method, dropping from 55 mpg to 46 mpg. Its city driving score suffers the most, dropping 20% form 60 mpg under the old method to 48 mpg under the new. Highway driving drops 12% from 51 mpg to 45 mpg.

The new EPA methods include the city and highway tests used for previous models along with additional tests to represent faster speeds and acceleration; air conditioner use; and colder outside temperature. MPG estimates will also be adjusted downward to account for factors that are difficult to replicate in a laboratory, such as wind and road surface resistance.

EPA has put a comparison tool on its Fuel Economy site to allow consumers to see an estimated outcome of the new method on older vehicles.

February 26, 2007 in Fuel Efficiency, Hybrids | Permalink | Comments (28) | TrackBack (0)

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Integers make the changes ugly.

For a car with an old rating of something like 50 MPG, keeping with integers only effects measurement by at most 1% due to rounding.

However, check out the results for cars with lower MPG. A car going from 11 to 10 MPG suddenly is 9% less efficient. But, it could be going from 11.4 to 9.6% (a reduction of 16%) or from 10.6 to 10.4 (a reduction of 2%). Integers add lots of "play" to the numbers when the denominator is so low to begin with.

Will these new numbers appear on the "stickers"? I am also curious if the stickers, for E85 vehicles, will also have E0gas and E85 numbers, since E85 has 72-75% of the energy value of E0. Will become a "severe sticker shock" to see a previous 40mpg vehicle go down to 20mpg, with the new epa calculations and the E85 gasohol penalty applied.

Although this will have a "detrimental" effect on fuel economy figures, I just realized that this will effectively make CAFE that much better. The fleet mileage numbers will stay the same, but with the actual mileage estimates per vehicle dropping, car makers will have to make up that difference to maintain their CAFE scores. Silver lining!

Andrew, Unfortunatly CAFE will still be calculated with the old MPG numbers. The new EPA ratings are not a back door CAFE increase. I wish it was.

If you go to the EPA web site, it also shows the old and new E85 numbers. Those viewing the E85 numbers will probably think long and hard before either buying an E85 equipped vehicle or actually filling up at the pump, if they can find one. Maybe the government could also "subsidize" the EPA numbers.

If you view the highway profile with respect to speed, acceleration, and deceleration on the highway, one might question whether the mode of driving is reasonable. If people actually drive that way, then I guess they can expect relatively poor gas mileage. I guess this isn't "real world" but I have gotten better than 55mpg driving at 65mph from Rock Springs, Wyoming to my home in Colorado in my Prius. But sure, if you drive as fast as 80mph and radically change your speed up and down then you can expect pretty crummy gas mileage, relatively speaking.

People still need to realize that the only reliable "real world" is their own. One cannot rely on the EPA to tell them what kind of gas mileage they can expect, especially for those vehicles that get the best mileage.

Just to show I am an equal opportunity basher (not really), we can blame Clinton for our current real world driving, that is, he did away with the 55mph speed limit, a mistake in my opinion. His wife is no better as she has refused to promise she would roll back the limits if elected President.

While we are all waiting for that silver bullet vehicle or silver bullet fuel, we can all slow down and do our planet and each other a world of good.


Stormy -

the problem with MPG is that the numerical values are fairly low, in the teens rather than in the hundreds. John Q. Public prefers dealing with integers, so inevitably accuracy is lost. Also, since it is a reciprocal measure of fuel consumption, relative improvements in terms of percentages are fundamentally misleading.

In Europe, manufacturers must now quote fuel economy in the familiar L/100km as well as the new gCO2/km. Presenting these numbers side-by-side for a number of years allows consumers to get a feel for what the new metric means for their wallet.

Expressing fuel economy in terms of specific CO2 emissions has the added advantage of permitting direct comparisons across fuel options such as gasoline, E85 and diesel. Moreover, metrics based on "equivalent" fuel consumption are arguably questionable in the context of PHEVs, BEVs and FCVs - but those can anyhow only be fairly compared on a well-to-wheels basis, which is beyond any one formal test procedure for the vehicle.

Still, it is unfortunate that EPA decided to stick exclusively with the old MPG metric in the context of introducing the new test procedures:

http://www.epa.gov/fueleconomy/label.htm

For reference, here is how you can estimate your vehicle's specific CO2 emissions, based on miles per US gallon:

gasoline: gCO2/mi = 9083 / MPG
E85: gCO2/mi = 6223 / MPG
diesel: gCO2/mi = 10218 / MPG

For example, the example sticker shown in the above link gives an estimate of 21 MPG average for what is presumably a gasoline vehicle. That translates to 433 gCO2/mi, arguably a rather high value. If you're in the market for a new car in the US, please look for a value well below 300.

t,

About a year ago I started keeping track of my gas mileage for every single tank. Interestingly, I found that I got a couple of MPG better mileage when I drove across Nevada at 4000 to 5000 ft elevation compared with driving near sea level in California. I haven't tried to parse the data to see if the difference was the elevation or the gasoline. With the phaseout of MTBE the difference may narrow. My guess is that there is a measurable difference in wind resistance at altitude. Would any of you ME types help me out on this? What is the difference in wind resistance for "mile high" driving? How much difference does it make in MPG?

Rafael,
I can only speak from aircraft "leaning" procedures. With a non-FADEC equipped engine, as one climbs to higher altitudes, the fuel mixture must be manually "leaned" to account for the decrease in atmospheric pressure and thus lower concentrations of O2. This leaning is necessary to maintain a proper stoichiometric fuel/air ratio. If you don't reduce the fuel mixture as you climb, you run "too rich-" thus burning excessive fuel. As you descend, one must do the opposite and increase the fuel mixture. If a pilot were to forget to do this as he was descending- the engine would run "too lean" and the engine's TGTs would rise- running the risk of torching your cylinders.

The ECU in our rather mundane automobiles handles this task. This would explain why you burn less fuel when driving at altitude. On the flip side- unless turbicharged or turbo-normalized, your engine also produces less power at altitude. But the unit power produced per pound of fuel burned is more favorable at altitude.

If you view the highway profile with respect to speed, acceleration, and deceleration on the highway, one might question whether the mode of driving is reasonable. If people actually drive that way, then I guess they can expect relatively poor gas mileage. -- t

Well, yes. If you drive the way the old EPA schedule expects you to drive, you will likely get the old EPA numbers. If you drive the way the Japanese 10-15 expects you to drive, your Prius should be averaging 84 mpg (35.5 km/l -- though I don't think that one accounts for battery draw). The idea with updating the EPA numbers is to make them more representative of the way most people drive.

You are right about speed vs consumption, but I can guarantee that just legislating a lower max speed is not going to work. We would need to convince the public for a need to do that, probably in conjunction with carbon taxes.

And just for anecdotal data points, I'd say that 90% of the Priuses I encounter on the highway are going flow of traffic (75-80 mph), some are going faster, and I've seen maybe two that were hypermiling (I'm picking on the Prius because the HCH, TCH, VW TDI are too similar to their "conventional" counterparts to identify at a glance, but I'll bet the ratios are similar for them. I don't see many Insights, but most of those do seem to going for economy). Now perhaps my experience is not representative, perhaps they go for economy at all other times, and perhaps, just perhaps, they're the ones who complain they don't get the EPA ratings.

The Saturn VUE in the graph shows just north of 10% improvement over their conventional ICE- VUE:hybrid catsup:salsa

FireStorm's Capabilities
First, let's look at what Krupa's FireStorm spark plugs give an internal combustion engine:
• More horsepower;
• 44–50% increase in mpg;
• Dramatic decrease in emissions.
Second, let's see what FireStorm plugs eliminate:
• Smog pump;
• Catalytic converter;
• Radio frequency interference (RFI) and the use of resistors in the centre electrode;
• Gap growth;
• Exhaust gas recirculation (EGR) systems;
• Misfire/hesitation/detonation/stutter and stumble

I drive a 1998 Toyota Camry. The OLD EPA numbers are 23 mpg in the city; 30 mpg in the highway.

The NEW EPA numbers are 20 mpg in the city; 28 mpg in the highway.

I have been keeping tracking my fuel economy since December 2005. Everytime I buy gas, I get a receipt and write down my mileage on the receipt. Then, I plug in all the data into my spreadsheet at home.

Since December 2005, I have averaged 30.8 miles per gallon in my Camry! Not only do I beat the new numbers, I beat the old numbers.

How do I do it?
(1) I drive between 65 and 70 mph on the highway. Once I see the sign for my exit, I move to the right (if I wasn't already there), and slow down to 60 mph.
(2) I anticipate red lights. If I see a red light in front of me, I let go of the gas and let the car coast as much as possible.
(3) I make sure the engine does not go above 3000 RPM. When I merge onto a highway, as long as it is safe, I accelerate very gradually.
(4) I drive the speed limit in the city! When it says 30 mph, I try not to go above 35 mph, to the chagrin of everyone who is behind me.

My next car will be a Prius, so I am getting used to driving efficiently...

I typically get around 34mpg in my Corolla, regardless of what the EPA says. That comes out to 267g/mi of CO2. Not bad.

the new prius numbers are def bs. what's more, i've gotten 30 mpg on my regular 97 honda accord ex while driving at a steady 75 mph with a carrier (!) on my roof (think about what that does to drag). old epa data: 29; new data: 26.
epa, give me a damn break.

To answer James' question about high elevations and fuel economy, the benefit is two-fold. The main one is actually reduced pumping losses. Gasoline engines waste a ton of energy creating vacuum in the manifold so as to run throttled. At higher elevations the ambient air pressure is lower, thus in order to achieve a particular absolute manifold pressure less vacuum is created, thus lower pumping losses. That's actually the main benefit. The secondary benefit is that at highway speeds, something like 80% of the drag is aero drag and only 20% is mechanical and tire drag. Since the air pressure is lower, the drag is lower and it thus takes less power to drive a given speed.

The old EPA method had been in use for 30 years. Why the change now? Because the Japanesse have hybrids in large number on the road and Detroit does not. The better accuracy agruement dosn't wash. "Your mileage may vary", the EPA numbers are accurate accross vehicles not accross drivers. I get better than EPA on my Focus, I could get better than EPA on a Taurus, but I couldn't get a Taurus to beat a Focus. The EPA numbers where accurate accross vehicles!

David,
I do all the things you do in order to save gas!!
Two additional things I do:
When driving in the highway, if there is a long downhill and if safe, I select NEUTRAL in the transmission.
I use energy tires (Michelin)

James and DieselHybrid,
Pilots do calculate the change in effective air speed as they increase altitude. For an altitude of 5000 feet the True airspeed (TAS) is 8% higher than the Indicated airspeed (IAS) as shown on the aircraft insturments. This is why aircraft in cruise fly as high as they can. IAS determines the aerodynamic performance of the aircraft. Thus if you fly your Cessan 172 at 105 knots IAS at sea level it is moving through the same mass of air as one flying at 105 knots IAS at 5000 feet, although the higher aircraft is moving through an 8% greater volume of air, covering 8% more ground distance (assuming same wind at both altitudes, which is not generally true).

For a non-turbo piston-engined aircraft, the fuel mixture is leaned as you climb since the mass of air sucked into the engine decreases. Otherwise the fuel/air mixture is too rich and you're just throwing fuel overboard. This means higher altitude equals lower power available, so this determines both the service ceiling and the airspeed you can maintain. Fuel leaning doesn't improve TAS.

FYI, here's the book performance numbers for a Cessna 172P to show the effect:

Cruise Performance @2400RPM
Alt. KTAS KIAS Fuel GPH
0 109 109 7.6
1000 108.5 107 7.45
2000 108 105 7.3
3000 107.5 103 7.15
4000 107 101 7
5000 106.5 99 6.85
6000 106 97 6.7
7000 105 94.5 6.6
8000 104 92 6.5
9000 103 89.5 6.4
10000 102 87 6.3
11000 101 85 6.2
12000 100 83 6.1

Thanks
Ed

comparing the EPA numbers to what's on green hybrid it seems that the EPA numbers are pretty good. With about 1000 prii (gen 2) in the sample the mean is 47 MPG.

When driving in the highway, if there is a long downhill and if safe, I select NEUTRAL in the transmission. -- Jorge

Depends on the car, but IIRC most modern fuel injection units cut off fuel flow under no load conditions, so this might actually increase consumption, at least a little bit.

comparing the EPA numbers to what's on green hybrid it seems that the EPA numbers are pretty good. -- tripp

That's a self-selecting audience, so I'd be a little careful. All the same, even if you don't use them for statistically rigorous numbers, they can still provide useful data.

cidi,

for the vehicle to cut the fuel injection typically the engine has to:

Be at operating temperatures.
Load (throttle position very near to zero).
RPM > factory set threshhold (for my vehicle it is around 2200rpm).

Therefore, my engine would only cut off the injectors if I put it in gear such that I am spinning the engine faster than 2200rpm. This "engine braking" will also slow you down. The amount of fuel used while coasting in neutral with the engine at idle while cruising downhill might be fine on a gentle grade where engine braking would slow you down enough to require throttle input to keep you going anyway.

30 vs 29 = ~ 3% better than EPA

Considering your odometer can be off by as much as 4% i would not get to excited.

If your car was getting exactaly what it was rated at by the EPA (29 MPG) you could appear to be getting 27.8 to 30.16 depending on which way and how much your odometer was off.


http://www.freep.com/apps/pbcs.dll/article?AID=2007702190383

Cidi,

that's true, but if anything you'd think it would over represent hypermilers and the like. The fact that the numbers are similar is interesting. Do you think the EPA numbers should be lower? Based on my experience and talking with other prius owners I'd say that 46 MPG is a pretty good round figure.

or the vehicle to cut the fuel injection typically the engine has to:

Be at operating temperatures.
Load (throttle position very near to zero).
RPM > factory set threshhold (for my vehicle it is around 2200rpm).
-- Patrick

I'm sure it varies from engine to engine. Mine cuts out at no load, even when stone cold (it's a diesel, so when it's cold you hear whether fuel is being delivered or not). Engine braking is also minimal with this car. If the speed drops such that the revs go below threshold, then yes fuel gets delivered (around 835 rpm when warmed up)

The fact that the numbers are similar is interesting. Do you think the EPA numbers should be lower? Based on my experience and talking with other prius owners I'd say that 46 MPG is a pretty good round figure. -- tripp

I also hear around 46 mpg, and that may well be realistic. But I still wouldn't use numbers that aren't independently verified for rigorous statistics.

I drive gasoline AT car with tachometer, and have installed simple air/fuel ratio display (simply deciphering signal from exhaust oxygen sensor). It allows me to know exactly when transmission shifts and locks/unloks torque converter, and also when fuel injection is cut-off and resumes on coasting, and when it is going into fuel overrich on hard asseveration. Helps a lot to adapt couple of fuel conservation modes, when I am not able to drive in a spirited way due to traffic.

One is quite simple. On hilly highway accelerate gently uphill – the key is not to provoke torque converter disengagement. Gasoline engine is more efficient on wider throttle. On downhill just gradually step off gas pedal. Torque converter disengages, engine RPM drops about 800 rpm lower, and fuel injection seizes completely. Engine still maintains slight engine braking which helps to avoid speeding ticket, and car’s inertia keeps engine spinning at RPM higher than threshold of resuming fuel injection – on my car it is 1200 RPM. It is safer, more convenient, and saves more fuel then switching to neutral on AT (by the way, switching to neutral on AT on downhill is illegal).

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