EPA MPG Estimates are Seriously Flawed

[imzjustplayin]

Originally Posted by theholycow

Manufacturers base their MPG on the very specifically documented tests that the US EPA requires (and the EPA spot-checks to keep them honest).
http://fueleconomy.gov/feg/how_tested.shtml
http://fueleconomy.gov/feg/fe_test_schedules.shtml

Originally Posted by Zalittle

I may be wrong about this, but I heard that vehicle manufactures base their MPG on a vehicle doing 55mph on flat ground. Which may be why under different conditions you get lower MPG.

theholycow is absolutely correct.. however it's my observation that if you cruise at around 50-55mph, you should be getting the CAFE highway fuel economy numbers... It may very well be a coincidence that cruising at 50-55mph yields the same fuel economy as the CAFE highway fuel economy numbers.

[theholycow]

I've never tried it but I'm not surprised that you can meet CAFE ratings simply by cruising at 55. Most people are not aware of CAFE ratings, though.

[slurp812]

Originally Posted by *************

theholycow is absolutely correct.. however it's my observation that if you cruise at around 50-55mph, you should be getting the CAFE highway fuel economy numbers... It may very well be a coincidence that cruising at 50-55mph yields the same fuel economy as the CAFE highway fuel economy numbers.

I believe they drive a "test circuit". Not just a steady speed.

[imzjustplayin]

Originally Posted by slurp812

I believe they drive a "test circuit". Not just a steady speed.

That's correct, they drive a simulated test circuit. If you look at the fueleconomy.gov website, they even specify how the simulated drive circuit works. http://fueleconomy.gov/feg/fe_test_schedules.shtml
However, like I said, it is my observation that if you drive a steady 50-55mph on the highway, you can achieve the CAFE fuel economy numbers despite the fact that the way those numbers are achieved are significantly different.

[kamesama980]

The manufacturers set the numbers but the EPA will pull cars at random (that aren't sold to consumers yet) and test them. if it doesn't match up, BIG fines (based on how far off and how many of that car made, plus tests of the rest of the line that could result in more failures)

Either for 2010 or 2011 they have raised the speeds for the highway portion of the test.

small cars get great city mpg, large cars get great highway mpg. case and point: my fiancees 2800 lb 03 mazda protege5 with an automatic with manual mode is rated 25/30. keeping up with traffic it gets 24 in town tops. I, trying, can get 30 mpg in stop and go traffic jams. it peaks at just over 40 mpg around 40 mph then slowly goes downhill. 32 mpg at 65, 30 mpg at 70, 28 at 75 and so on. compare to my moms 3800 lbs 02 buick lesabre that might get 20 mpg in town with someone more patient than me driving but gets almost 35 mpg at 65 with 2 passengers and a weeks worth of luggage. Alas I don't drive it enough (ever except family vacations) much less with a scangauge to know what it gets at other speeds.

[imzjustplayin]

Originally Posted by kamesama980

The manufacturers set the numbers but the EPA will pull cars at random (that aren't sold to consumers yet) and test them. if it doesn't match up, BIG fines (based on how far off and how many of that car made, plus tests of the rest of the line that could result in more failures)

Either for 2010 or 2011 they have raised the speeds for the highway portion of the test.

I thought that was for/in 2008 where they did the testing up to 80mph? Also, what I find interesting about these test procedures is that I don't think they're actually conducted but instead are actually mathematics being applied to the existing CAFE standard test procedure. To calculate highway fuel economy for the 1985-2007 EPA test cycle (monroney stickers) multiply CAFE highway fuel economy by .78 and round out the number down to single digits. To calculate city fuel economy, do the same but multiply by .9
instead.. I tried figuring out the math for the 2008+ fuel economy numbers but I wasn't getting any sort of consistency which leads me to believe that there is a bit more to the test procedure for the 2008+ fuel economy numbers that aren't reflected in the CAFE numbers since CAFE test procedures don't specify the usage of the A/C.
edit: I found this site which apparently explains the changes that DO occur in 2011 which affect the way fuel economy is calculated for the Monroney sticker.. http://allaboutfueleconomy.com/EPA/EPA_Info.aspx Pretty good that I was correct in knowing that city fuel economy was .9 of cafe and hwy fuel economy was .78 and didn't even know it until I stumbled upon this website.

Originally Posted by kamesama980

small cars get great city mpg, large cars get great highway mpg. case and point: my fiancees 2800 lb 03 mazda protege5 with an automatic with manual mode is rated 25/30. keeping up with traffic it gets 24 in town tops. I, trying, can get 30 mpg in stop and go traffic jams. it peaks at just over 40 mpg around 40 mph then slowly goes downhill. 32 mpg at 65, 30 mpg at 70, 28 at 75 and so on. compare to my moms 3800 lbs 02 buick lesabre that might get 20 mpg in town with someone more patient than me driving but gets almost 35 mpg at 65 with 2 passengers and a weeks worth of luggage. Alas I don't drive it enough (ever except family vacations) much less with a scangauge to know what it gets at other speeds.

Small cars generally get better fuel economy in the city because they weigh less and have smaller displacement motors. Motor displacement is extremely important in city driving due to the excessive amount of idling these cars have to do and cars with large motors idle at a GPH rate that is twice that of smaller cars. Small cars also typically have lower powered motors and in a bid to make the cars peppy, they give them gear ratios that make them scream on the highway whilst not consuming too much fuel but still in their power range so people can quickly accelerate w/o changing gears.

[theholycow]

Originally Posted by *************

edit: I found this site which apparently explains the changes that DO occur in 2011 which affect the way fuel economy is calculated for the Monroney sticker.. http://allaboutfueleconomy.com/EPA/EPA_Info.aspx Pretty good that I was correct in knowing that city fuel economy was .9 of cafe and hwy fuel economy was .78 and didn't even know it until I stumbled upon this website.

Very interesting...

In short, as I read it:

• There were originally two test cycles. The Monroney sticker displayed .9 and .78 of those tests.
• For 2008 they added 3 more test cycles. They allowed manufacturers to still only actually test the original 2 test cycles and calculate the other 3 from that data.
• For 2011 they require manufacturers to demonstrate that their calculations are similar to what the actual result would be with the additional 3 test cycles.

I'm not sure what the effective difference is for 2011. Since they already audited random vehicles, what is the new requirement? Is it that now they will revoke calculation privileges from manufacturers whose calculations don't match actual tests?