First reason and this would apply to highway driving is the weight of the transmissions. Less weight equal better economy and manuals weigh a lot less then automatics. Second reason is when you shift into the highest gear, lets say fifth, you stay in that gear, but an automatic transmission comes in and out of overdrive/drive and locks and unlocks the torque convertor depending on terrain, wind and traffic.

The therory is that if you drive economically with the manual transmission and shift at low RPM's your mileage will always be better then an automatic.

 

I think the addition of a 6th gear would help the most, something that would run the engine ~2000 RPMs at 70 MPH

Sadly 5th gear (on mine) seems optimized for 45-50 MPH. Resetting the Economy counter before cruising at those speeds for ~5 miles can net me 40-45 MPG.

With another gear one could drive across the country at 70 MPH and still get 40 MPG I bet

 

Things like electronic control and lockup converters have much improved the efficiency of auto trans in recent years. Still, you have hydraulics in place that rely on pressure provided by a pump (read: HP loss). Driven sensibly, a manual will still get better MPG than an auto, all else being equal.

Given the original information, a 3 MPG gain is easily attainable. Chevy has likely spec'd a tire with less rolling resistance and has tweaked the spark tables for better economy. They may also have gone slightly lean on the calibration, although that's difficult and still be able to meet emission standards when the AFR deviates from stoic.

I tuned my HHR and easily exceed the published (old or new) specs by more than 3 MPG, but I also hate it when people start trying to compare mileage, since there are so many variables that make comparisons moot.

 

I wonder when they're going to strip down the Aveo to plastic seats, donut tires, and no A/C so they can get 50 MPG...

 

If high mpg numbers are the goal, GM should look to clean diesel. Admittedly, diesel prices right now are a little crazy, but diesel returns more efficiency than gasoline. The wife's 2005 Beetle TDI (5-speed manual) got 49.6 mpg on her Nashville trip last weekend, and that was pacing with traffic around 80mph cruise ctrl active, and through hilly western KY & TN. She said she started out trying to drive 75 but was practically getting ran over, so she upped it to keep better pace w/ everyone else. Gasoline version of the Beetle only gets slightly better than half that.

In keeping with the topic of this thread, I will add that her Beetle uses full synthetic oil (has to for warranty, use VW spec 505.01 compliant oil which is full synth diesel oil), and the OE tires are 205/55HR16 Michelin "Energy" MX which I suspect to be a low rolling resistance energy saver tire from Michelin, and I bet the Cobalt XFe OE tire may be the same type.