Answer from Fueleconomy.gov on 2015 6.2/8 speed

[Ashtonman]

I was wondering why the math on the 2015 did not average correctly, so I emailed them and this is the very detailed answer I received.

Based on the answers I received I think the 2014 combined average is overly optimistic and the 2015 is going to be closer to reality.

(I better not forget “IMO”)

 

Email from fueleconomy.gov

 

Thank you for contacting us. The EPA combined average is a tricky business because it is not a simple arithmetic average and is not based on the whole numbers for city and highway that are shown on the window stickers. The EPA calculates the combined average from the unrounded (technically it is rounded but to 4 decimal places) City and Highway values and then they use a weighted (55% city and 45% Highway) harmonic mean and banker’s rounding. The formula is:

 

1/(.55/City + .45/Hwy) where City and Hwy values are to 4 decimal places. Below is the math for the two models you noted:

http://www.fueleconomy.gov/feg/Find.do?action=sbs&id=35818&id=34501

 

The unrounded numbers for the 2014 GMC Sierra 4WD 6.2L, 8 cyl, Automatic 6-Spd are:

City: 14.3721 (Column F - City08U from data at: http://www.fueleconomy.gov/feg/download.shtml File at the top for all years)

Highway: 20.2147 (Column AJ - highway08U)

 

So the math works out as follows:

1/(.55/City + .45/Hwy)

1/(.55/14.3721 + .45/20.2147)

1/(.03826859 + .02226103)

1/.06052962 = 16.5208 or 17 Combined – Just barely rounds to a combined rating of 17.

 

The unrounded numbers for the 2015 GMC Sierra 4WD 6.2L, 8 cyl, Automatic 8-spd are:

City: 14.7564

Highway: 21.1260

The math works out as follows:

1/(.55/City + .45/Hwy).

1/(.55/14.7564 + .45/21.1260)

1/(.03727196 + .02130077)

1/.05857273 = 17.0728 or 17 Combined

 

To further complicate matters, the manufacturers also have the right and are encouraged in fact to voluntarily lower their City or Highway values if they have any reason to believe that the EPA test values may reflect a higher value than their expected average customer. They are not required to provide any rationale so you will sometimes find numbers between model years that change even though the vehicle is mechanically identical which gives the impression that an error was made when in fact it has to do with how the rules are applied. I hope this addresses your concerns.

Sincerely,

www.fueleconomy.gov

[Coppertone]

Wow, very interesting.

[Daryl LBZ]

Typical BS as usual.

[fondupot]

Typical BS as usual.

 

Exactly. The difference in mileage will be negligible.

[f8l vnm]

Any news on the gearing ?

[TxTruckMan]

What were you expecting? GM said a max of 5% mpg improvement. That's 1 mpg max. We've hit the point of diminishing returns from tranny gears. The new 10 speed may be even more disappointing for even more money. I'm glad I have the 6 speed.