Car and Driver: These Tests Failed You: Why Is the EPA So Bad at Estimating Hybrid Fuel Economy?

This is what I think I know:
1975 to 1984 EPA = CAFE
1985 to 2008 EPA = CAFE with fudge factor (0.90 city fudge factor) (0.78 highway fudge factor)*
2009 to Present EPA = 5 tests including 3 new tests and the same old 2 CAFE tests. But you are allowed to estimate the 3 new tests.

*Thanks Fuzzy1

 

They have the same test, but the calculation is different. The 60/51 gen II pre 2008 epa, I think works out to about 66 mpg cafe. The current prius is 70 mpg. An toyota engineer predicted the gen IV would get 90 mpg cafe.

 

That makes sense. Thanks

 

Is there any evidence that for non-hybrid, non-turbo cars the three "estimated" tests give accurate numbers?

I'm OK with making hybrid and turbo car actually do all five tests. But how expensive can it be for all cars to do all 5 tests?
I'm guessing (without evidence) that it would be a lot less expensive than all the wind tunnel tests to see if the models and prototypes perform the way theory says they should.

 

No there is not, what we do have is anecdotal evidence that they do. The city and highway tests no longer reflect our city and highway driving, and could be revamped. Automakers do the tests, and if the tests are more reflective of american drivers, then they will build cars that more accurately reduce fuel usage. That probably means using crappy winter blend E10 on the tests also

All cars should have to do all the tests. I'm guessing the car makers do all 5 then pick which ones to submit if they have a choice. Tesla clearly stated that they were required to do all 5 and it reduced mpge. I have no problem with cafe favoring hybrids, but we should not be putting the numbers on cars for buyer information. All 5 tests should be available on-line.

 

Sorry but none of these work for me. All I want in every owner's book:

• Standard Day, 60-70F, dry, paved, level road, car in "as sold" configuration, a chart showing MPG as a function of MPH for two, 10 mile segments, at opposite directions or around a track at cruise control steady speed:
  • 10 mph
  • 20 mph
    • minimum cruise control speed
  • 30 mph
  • 40 mph
    • hybrid-special or critical speed(s)
  • 50 mph
  • 60 mph
  • 70 mph
  • 80 mph
• Density-altitude correction chart showing altitude and temperature correction factors
• Energy vs ethanol chart showing the effects of E0-to-E85
• Warm-up time to hybrid efficient mode at 25 mph
• Hill climb test, fuel consumed as a function of speed up a constant 8% grade, 1,000FT climb/200M:
  • 50 mph
  • 55 mph
  • 60 mph
    • identify any specific speed that begins to drain the traction battery
  • 65 mph
  • 70 mph

I have done these ad hoc tests over the years for both of our Prius and often shared the results here and other forums. I needed this to know how to drive our cars efficiently. If I can do this for my own Prius, why can't anyone else?

Of course they could and added to the Owner's Manual, we'd finally have credible data to understand the performance we should expect. Heck, a bright kid could write a Java app, web based, that with a GPS track from say a Garmin could make a reasonable estimate of:

1. Total trip MPG
2. Highlight specific areas where there may have been avoidable energy waste

But then I'm a non-current pilot rebuilding an airplane to return to flying and such things come naturally to me.

Bob Wilson

 

As pointed out there are not 'new' and 'old' CAFE tests; just different fudge factors, and as of the latest revision, new tests for the car sticker that do not apply to CAFE.

I would also like to point out that the fudge factors may over-correct actual testing rather than under-correct. Show me the fudge.

CAFE results: CAFE 2008 -- by vehicle | PriusChat

 

Years after the EPA established their testing and rating system, many of the superhighways have raised their maximum speed limit to 70 mph and 75 mph. This means many are going +70 mph will see a sharp drop in fuel efficiency.

 

So, here are a few questions related to why EPA might be bad at estimating real world MPG:

1. Do the EPA tests allow the use of 100% gasoline (no ethanol)?

2. Since most of us (I think!), drive on 10% ethanol, do the EPA fudge factors account for that?

3. What is the difference in available energy per gallon of pure regular gasoline vs. E-10?
(As an answer to this, I would like a number something like you see in the statements that diesel fuel has about 12% more available energy per gallon than unleaded gasoline, only the number will be less, not more.)

Thanks

 

The epa use special gasoline, with no ethanol. They attempt to compensate with a fudge factor, but this was set in 2008, and ethanol content has risen since then so that everyone is using e10 now, average in 2008 was about 5% ethanol. Different places like polluted cities had e10 the entire time. E10 has approximately 97% of the energy of E0, which is likely around 1.5 mpg in a car like the prius. Diesel has about 15% more energy than E10, and summer gas has more energy than winter gas.

 

So, going off of austin's figures:
austin says that EPA attempted, as of 2008, to "fudge factor" in E-5, but today we are using E-10. That SHOULD mean that the EPA figures are what the EPA estimates real world mileage should be using E-5, not pure gasoline.

If the real difference between pure reg. unleaded and E-10 is about 3% of energy, the difference between E-5 and E-10 should be about 1.5%, or about .75 mpg on an approx. 50 mpg Prius.
So, if I add the un-fudged-for decrease to my current 46.9 mpg (over 55K miles, hand calculated), I would get about 47.6 mpg if I used E-5, pretty darn close to the EPA estimate 48 mpg highway, which is my target because so much of my driving is highway (65+mph), and most of what I call city driving is stop and go to an extent that I expect even the Prius to show a negative effect.

That's not too bad for the EPA estimates, and I am not a hypermiler and I am a generally clumsy guy who doesn't treat much of anything too gently.


BTW, what this MIGHT mean is that--assuming E-15 becomes a standard for real world driving, as some of the standard setters hope--fudging for E-15 might get EPA numbers closer to real world numbers for all types of cars.
I know the carmakers wouldn't like to see their numbers lowered, but it would make folks happier after the sale, so maybe it's a wash...
However, this won't deal with the special problems of estimates for hybrids and turbos, for which I think the only solution is to have cars do actual tests on all five cycles, before applying the various fudge factors.

 

If one is running E85 (if you can find it), does one ignore the ethanol burned and just use the 15% gas burned?

DBCassidy

 

Yea ... and the 2 seater GenI Insight came in at a whopping 70mpg highway 61mpg city !!
I had no problem beating the old EPA ... but it wouldn't be a bad idea to simply have a hard core stop & start element of the test for 5 miles, on a rainy 35 degree morning . . . . then a constant 60mph on flat ground part of the test, warmed up, for 5 miles. That'd be the worst & the best. No surprises. You may do better then the test's easy part, but you be much harder pressed to do more crappy than the hard part of the test.

 

For the ethanol content and the energy available, I would have worded it like this. Ethanol has approximately 70% of the energy of gasoline. This is how the 3% drop for E10 comes about: 100% energy from 90% of the fuel and 70% of the energy from 10% of the fuel, so 90 + 7, or 97%.

That's too easy, though. So looking at E85, it should be about 100% of 15% and 70% of 85%, so (1 *.15 = .15) + (.85 * .7 = .60), or simply 15 + 60 = 75% of the energy of straight gasoline. I use these numbers in my off-the-cuff calculations to see if E85 is overpriced.

 

That's one of the things I wanted to know, but was too lazy to figure out for myself. Thanks, Mark.

Now, I'm going to assume that, at ethanol levels for which the engine is rated, both regular Otto-cycle engines and Prius Atkinson/Miller engines use ethanol about as efficiently as the hydrocarbons in gasoline. Right?

 

In general, it is probably a safe assumption for these calculations. That is until the ethanol content starts to climb. With an octane rating of 105, the ethanol will raise the overall octane of the blend. Depending on the engine's compression ratio and how much it can adjust its timing, it might be able to burn the higher ethanol fuel more efficiently.

Studies looking at the efficiency differences of varying ethanol blends show that, while the volumetric(miles per gallon) efficiency drops, the energy content(miles per btu) efficiency can raise for some cars with increasing ethanol percentages.

Going back to EPA test gasoline. For federal emission cars, it is 93 octane. For 50 state ones, it's 91. With the stigma a mainstream car gets for calling for premium fuel here, manufacturers will label such a car for regular to avoid the loss of sales from it. A regular fuel labeled car that can actually take advantage of higher octanes could end up getting better results on the test because of that octane difference.

 

In this connection, this appeared today:

Ford under fire for its hybrid fuel economy numbers - The Globe and Mail

 

The EPA is truly awful. They rate the C at 46 highway. Today I drove from Lansing to Ann Arbor and got 62 MPG.