I have to wonder what kind of fuel economy an impala would get if it were optioned with a 4.5L V8 turbo diesel (that has AFM and VVT?) backed by a 6L90E and 2.41 gears? Maybe even slip that in a streamlined wagon body with the ability to tow 5000# I'd dump my truck for that in a minute!

And chances are, I'd also pick up a fancy gas SS for the wife

 

For the record, a 2004 GTO with the LS1 and manual tranny was EPA rated at 17 mpg city and... 29 mpg highway.

Jump to the LS2 and an extra 50 horsepower and that number drops to 16 and 25.

The 300 horse Impala SS gets 18 city and 27 highway.

1) 30 plus MPG on a V8 is not only possible, but can easily be done without much effort.

2) Regarding highway fuel economy, improved aerodynamics can compensate for weight. Weight does affect city mileage. No amount of aerodynamics counters that.

3) Once you start going over 350-400 horsepower, forget about great fuel economy. Sure there's DOD and other fuel cutoff functions to counter the "city" EPA cycle, and maybe even the highway cycle (when the EPA changes it's test proceedures, automakers will simply change their drivetrain programing to compensate). But you still will be drinking plenty of fuel.


Personal experience.

My frequent road trips in my '97 LT1 6 speed Z28 almost always equaled around about 30 mpg at a steady 75-80 mph. My B4C automatic, by comparison, is a gas hog. Same type of driving doesn't even give me 25.

I'm sure the drive ratios and the way I shift (unless I'm in a hurry or putting the smakdown on someone, I tend to go in high gears sooner and leave in the highest gear possible) has some effect, but there's more to things than just weight.

 

Just for informational purposes. I got 22.5 MPG in my 6 speed 99 SS coming back from Springfield, MO to St. Louis. It is a hilly drive. The car was cammed, had a McLeod Twin Disk, 4.10 gears, and we were cruising north of 80 for a long stretch. I was pretty impressed (especially since the exhaust leak had her running rich).

 

Wasn't there a change in the way they rated mpg from 04 and 05? HP doesn't necessarily change fuel economy. It still takes the same amount of hp to keep an 04 GTO cruising at 60mph as it does an 05. The larger displacement engine probably didn't help, but I can't see it being responsible for that big of a drop.

I don't think 30mpg on the highway in a v8 sedan is as easy as everyone an this board is making it out to be, especially with the new EPA rating system that will come into effect (if it hasn't already).

 

That 29 mpg highway # for the 04 GTO is an anomoly. Either they hit some magical sweet spot in the gearing or someone flat out forgot to carry the 2 or something because the LS2 gets virtually the same fuel economy (-1 in the city) on both the 04 to 05 Corvette changeover and the 04 to 05 SSR changeover. In fact the LS1 GTO and the LS2 GTO have the exact same ratiings with the automatic-16/21.

Highway cruise speeds are maintained at far less than maximum HP (<100). This is why displacement on demand is possible, but its also only results in an 8% increase in fuel economy) The LS2 does not burn any more fuel than the LS1 to generate the same hp while cruising. Yes, it takes more air and more fuel to generate higher maximum hp, but these tests are not run at wide-open-throttle.

Fuel economy data available here- http://www.fueleconomy.gov/feg/findacar.htm

 

2004 real world mileage is no where near 29. I average about 20-21 in my daily commute in the Chicago area.

 

Depending upon when and where you're driving, that's not exactly "highway" mileage. I get 20-21 with my A4 '04... my friends with 6-speed '04's get between 25 and 28 mpg on the highway with their GTO's...

Also, don't forget you probably run reformulated gas with at least 10% Ethanol... decreases your mileage by 2-3 mpg...

 

If he is in Chicago it is for sure 10% or more, all pumps in IL must be at least 10% ethanol (says so on the pumps).