McGuireV10

As everybody probably knows, starting in 2008 the EPA uses a more complex method to figure window-sticker mileage. You can compare the official numbers for older vehicles to the numbers that the new method would produce using this site:

http://www.fueleconomy.gov/

One thing I found odd about the ratings is that my "FlexFuel" E85-capable 2007 Suburban LTZ 4WD gets a better rating (running on regular gas) than the non-FlexFuel version. This is true of both the old-method and the new-method calculations (although predictably, all numbers go down under the new method).

Gas-only engine (non-FlexFuel):
old: 14/18
new: 13/17

FlexFuel engine (running on gas):
old: 15/20
new: 14/19

I'm curious about this. I've never been able to find much specific information about what is different about FlexFuel engines. I know in general what E85 requires -- at a high level, better fuel pumps due E85 being thinner and not cooling as well as gasoline, and probably stainless steel lines since E85 eats rubber, very different filters, and higher-flowing injectors -- but I'm wondering what about this system should affect mileage for regular gasoline -- particularly since the FlexFuel changes sound like they'd add a lot of weight.

I actually thought about posting this in the Tahoe section for greater visibility, but confusingly, the Tahoe 4WD data is presented in a completely different format that doesn't make it possible to compare FlexFuel engines with gas-only engines. (Your tax dollars at work!)

ZX1100F1

This discrepancy that you found is just that, some of the mileage figures have not been updated correctly to the new 2008 EPA MPG ratings.
The LC9 (Flex fuel) and the standard LY5 5.3L (2WD only) engines achieve the same MPG and performance numbers in testing which was 14 City/20Hwy on the old standard, they will both have the same ratings once the new standard is fully in-place and the data is tallied.
You also need to keep in mind that some people are experiencing fuel economy that is better then the Monroney sticker’s EPA estimates on the old system, I currently drive a new 2007 CC10703 (Reg Cab 2WD Short Bed) with the 4.8L which was rated 15 City/20 Hwy and consistently attain 20.5+mpg Highway with the cruise set 75mph.

McGuireV10

Sure, I don't put much stock in the numbers for real-world conditions -- I'm getting better highway numbers, too -- living in flat Florida probably helps a lot. I just found it odd that their supposedly carefully-controlled tests showed better FlexFuel numbers across the board. The 4WD Tahoe is shown in some completely different format that doesn't allow Flex-to-gas comparisons, but the 2WD Tahoe and the 2WD Suburbans both show the same results -- higher FlexFuel numbers.

SNO

The discrepency might be that you aren't looking at the same engines. If you were comparing 4WD 'burbs, the "gas only" engine is 6.0L whereas the flex-fuel engine is 5.3L.

2004urb

The difference is in the power delivered by ethanol.
Just an fyi, gallon for gallon,diesel is 30% more powerful than gasoline,
and gasoline is 30% more powerful than ethanol, hence more fuel is required
for the same power when using ethanol blends, E10/E85, and even more so
when using straight ethanol. To get the same cost per mile, ethanol has to be
cheaper, by 30% if using straight ethanol.

McGuireV10

No, I was talking about a discrepancy in the numbers using the same fuel, the difference being non-E85-capable engines compared to an E85-capable one -- but both running gasoline. I believe SNO identified the reason.