Our 2010 LT now has about 1300 mi. on it. The fuel mileage is very disappointing - only about 20 - 21 mpg. I thought at first it was because it was new, but after 1300 miles it should be "broken in."
I don't think we can blame the driver because before we bought the 2010 we had a 2009 rental Impala which averaged 26 - 27 mpg. combined driving. I naturally assumed the new one would be about the same. And our previous car (3.1L Buick Century) was very consistent at 30 mpg. highway.
So, any thoughts why our new car is getting such poor mileage?

 

First off is the city or highway? also how are you calculating your MPG?

 

Also your going from a 3.1l to a 3.5 or 3.9. You are going to get worse MPG with a bigger engine.

 

The driving is combined - some city, some highway. It's the same kind of driving we were doing with the '09 rental Impala that averaged 27 mpg.
But it doesn't seem to matter - some tanks of gas were mostly city and a couple were mostly highway but it comes out to about 21 mpg no matter what.
I'm using the car's computer. I reset it every time I fill up so, no, it's not averaging lots of different driving.

 

The cars computer is never correct. Try doing this:
Fill up
Clear your trip
Drive till you have to fill up
Fill up and take your trip number and divide by gallons of gas it took to fill it up.

 

New here and just chiming in 1300miles is not enough for catalyst break in there was a infomation bulletin regarding this keep driving it, it will get better

 

1,300 miles is not nearly enough for it to be broken in. I've had 14 new company vehicles and most have gotten the best mileage after 12,000-15,000 miles.

 

^^^^^^^

 

Thought you might like to read "the rest of the story."
In mid-October we headed down I-71 and I-75 to our trailer in SW Florida. The starting mileage was about 3150. We averaged 22 - 23mpg on the trip down and that was 100% highway driving. The speed limit on I-75, once out of Ohio is 70mph so, of course, we drove at about 75 - 80 to match the traffic.
In Florida, besides the usual local trips for shopping, visiting, etc. we also did some highway driving, usually at speeds around 75mph, to places like Naples, Sanibel, Fort Myers Beach, and Sarasota. And the gas mileage started to improve. By the time we were ready to head home for Thanksgiving, we were in the 25 - 26 mpg range. As we headed north on I-75 each fuel stop got better until our final fill-up in Ohio where we hit 31 mpg! At that point we had 7155 miles on the car. So, apparently it takes about 7000 miles and a lot of highway driving to finally attain the EPA mileage. I don't understand why there should be that long of a break-in period.
I also learned that the computer is pretty consistently 0.4mpg higher than the calculated mpg so I now rely on the computer and mentally adjust for the 0.4 difference.

 

Ya gotta keep your foot out of it! Drive like I do (geezer) and you'll get 23 in town and 32 on the road. A K&N Filter wouldn't hurt either.