I don't know because if you look at the Saturn Aura 4cyl. and the Solstice with an auto and the same engine, the Aura get almost 3 mpg better and the car is a few hundred pounds heavier. In the HHR the Turbo 2.0L gets the same hwy mileage as the 2.4L

Based on all that (Aura 2.4/3.6 and HHR) I'd estimate that a turbo 2.0L would get ~3 mpg better than a 3.6L

 

The problem with making estimates based on current cars is that gearing can make a huge difference in EPA numbers -- on a manual. On an automatic, shift points can make the same sort of difference.

So it's hard to extrapolate accurately when comparing something like a Sky/Solstice to an Aura, because the Sky is likely not optimized for fuel mileage, while the Aura is.

 

The other problem is EPA "estimates". When are they actually going to test the cars? I know my previous Vette pulled down 30mpg on the highway, but was rated lower. Why don't they actually drive the cars till they run empty, put in 5 gallons and then drive the car again on a test track till it runs empty. Average the mileage driven for the 5 gallons and get a true rating of what the fuel mileage is at highway speeds.

 

They do actually test the cars in very controlled conditions on a specific test cycle. So they're already doing better than you ask for above.

The 'estimate' comes in because they're trying to predict your results from their test. And you don't drive like they do.

For example, the highway test is not 100% highway, so it's entirely possible to beat their results.

 

Correct, it is also possible to go the other way. If you remote start your car or drive your Camaro around in 1st just to hear the sweet rumble your will get much lower mpg

 

Only if you read Deforestation is a huge problem...you can't read anything about CO2 levels and global warming harldy without mention of deforestation. Trees are CO2 sinks.