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Discussion Starter · #1 ·
The EPA has their efficiency ratings for the 2012 Volt up on FuelEconomy.gov, and it's showing a highway "Electricity Only" efficiency of 93 MPGe -- up from 90 MPGe for MY2011. This bumps up the combined city/hwy efficiency from 93 MPGe to 94 MPGe.

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

I had been under the impression that the 2011 and 2012 models were more-or-less identical.

Does anyone who's in-the-know have an explanation for the improvement in the rated efficiency for MY2012?
 

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How about the fact that the 2012 base car is a little bit lighter weight (less options)..............note that the difference is in the HWY mpg.
Maybe in 2012, they actually drove the vehicle in Regular 'drive' mode instead of Low gear? i'm betting on lighter weight!
 

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Discussion Starter · #3 ·
How about the fact that the 2012 base car is a little bit lighter weight (less options)..............note that the difference is in the HWY mpg.
Maybe in 2012, they actually drove the vehicle in Regular 'drive' mode instead of Low gear? i'm betting on lighter weight!
That's an interesting thought. They did get rid of the lightweight Bose sound system as a standard option for 2012, which would add some weight. But they also got rid of the Nav and audio hard drive, which would reduce weight.

However, if the improvement was due to weight reduction, you'd expect to see a bigger impact on City driving than Highway. When cruising, higher weight increases rolling resistance, but rolling resistance is a smaller contributor to losses at higher speeds than at lower speeds. (see here: http://webarchive.teslamotors.com/display_data.php?data_name=range_blog4 ) And in City driving, you have a lot of stop-and-go driving, where weight is most important.

And regarding D vs L mode, it shouldn't make a difference for Highway efficiency when you're just cruising.
 

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Perhaps it could be that the relative price of gasoline and electricity varied and that affects the equivalent mpg?
Wouldn't that effect city and highway rating equally? Only the highway rating went up.

And, to add more FUD to the discussion, the yearly operating costs didn't change in the rating, nor anything else. Only the highway rating went up, and I don't see how that wouldn't affect anything else. So I'm thinking something's half baked here.
 

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Perhaps it could be that the relative price of gasoline and electricity varied and that affects the equivalent mpg?
No. MPGe is is based on how far the car will go on 33.7 kWh of electricity. The cost of the electricity doesn't matter.

However, the cost for driving 15,000 miles didn't change despite the increase in MPGe. It should have gone down by seven bucks or so.
 

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Discussion Starter · #9 ·
Perhaps it could be that the relative price of gasoline and electricity varied and that affects the equivalent mpg?
MPGe calculations aren't affected by fuel prices. It's just uses the chemical energy in a gallon of gasoline (33.7 kW-hrs) to convert between wh/mi and MPGe.

Perhaps it is the EPA test methodology that changed.
Yeah... unless someone can come up with another theory, that seems most likely at this point. Maybe the EPA tweaked one of the "fudge factors" that they use to determine efficiency for EV's(?). Unless anyone knows something more concrete, I guess when the 2012 Leaf numbers are posted, we can compare those to the 2011 numbers to see if there's a similar jump in highway efficiency.
 

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My guess would be software updates and changes in calibration help improve efficiency.

I would also guess that software updates to the 2011 will improve things a bit as well.
If that is true then DonC's assertion that the yearly costs would be slightly lowered would hold true as well wouldn't it? How can you have better overall efficiency, for whatever reason, and not have correspondingly lower operating costs per year?
 

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The difference is due to revisions in the engine calibration (one more year of tweeking by the development engineer), test accuracy (easily 2-3%), vehicle-to-vehicle (could be as high as 10% depending on vehicle agging history) and driver-to-driver (could be an additional 5%) variation. So a change of 3% in the highway number is really not a change at all. Note most if not all manufactures pick the good vehicles to submit to EPA!
 

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I know the test is done on a dynamometer. There is still test variation. I have been in the Emissions testing area for over 30 years. The dyno test takes out some enviromental variation but not driver or vehicle differences. EPA even acknowledges this by one of their test pollicies. If they elect to confirm a manufactures test, they consider the test valid if the fuel economy is within +- 4%.
 

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Discussion Starter · #16 ·
Well, despite the various guesses, I'm pretty sure this is just a mistake and nothing more. (unfortunately.)

After thinking about it for awhile, I remembered that MPGe is just a derived value calculated from the electric efficiency. And the posted values for kW-hrs/100 miles didn't change at all between 2011 and 2012. (See here: http://www.fueleconomy.gov/feg/phevsbs.shtml )

The 2012 highway electric efficiency is still 37.4 kW-hrs/100 miles. 1/37.4*100*33.7 = 90.1 MPGe. If it was actually 93 MPGe for 2012, we should see a corresponding electric efficiency of ~36.2 kW-hrs/100 miles.
 
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