I recently (July 2012) bought a 2011 Honda Fit because it seemed to offer many features I would enjoy including good use of interior space and great
gas mileage at 33 mpg highway, 31 city driving. I thought to continue with manual transmission because I thought it was better on gas mileage, gave me more control of the car and I was used to the manual now since I have been driving a manual since 1985.
I have been watching the gas mileage continuously decline almost daily and now its at 29.6 mpg, city driving. Is there something I can do to bring it back to the Manufacturer’s approximation of 31 and/or improve this gas mileage. Is it my driving since i had been used to the “rougher” Honda Civic DX '94 which I had driven for 14 years? Any kind suggestions would be greatly appreciated. I’m really disappointed 
There is nothing wrong with the mileage you stated…29.6 since all gas mileage estimates are just estimates. You should feel lucky to get as close to the estimate as you did. It takes sophisticated digital equipment to check your true mpg anyway and I bet you are using the old pencil and paper method of determining mpg. That method will get you close but it’s far from accurate.
The car is rated for 27 MPG city and 33 MPG highway. As you can see here
http://www.fueleconomy.gov/feg/PowerSearch.do?action=noform&path=1&year1=2011&year2=2011&make=Honda&model=Fit&srchtyp=ymm
There’s likely nothing wrong with your car.
You might check the air pressure in your tires to be certain that it matches what is called for on the sticker on the door post pillar. The weather in your location may have turned a little cooler at night so that the car doesn’t warm up quite as quickly and the ECM enriches the mixture.
I notice all kinds of variation in gasoline mileage on a 25 mile trip we make once a week. My wife likes to eat at a Cracker Barrel restaurant, and being an old country boy, we have one that we can reach by country roads so that I can look at the fields. I often fill the gas tank before we leave and note the gasoline mileage reading on the display. The gasoline mileage on our Sienna will vary from a low of 21 mpg to a high of 26 mpg for these trips. If the mileage going up is at the high end of the scale, then I’ll celebrate by having dessert. If the mileage is on the other end, then I skip dessert.
How many miles are on your Fit? His was the maintenance prior you getting it? I do have a same car, a 2011 Fit with a manual transmission and I am getting 39 mpg in the city and around 44 on the highway.
If the maintenance up to date then the only thing can affect your mpg is the way how you drive it. I am shifting it into 5th gear at 40-45 mph, never speed all the way to the red light and jump on the brakes and I do tend to stay the right hand lane on the freeway.
The engine itself is really rev happy and for your information (not in the owners manual) the i- vtec kicks in around 3300 rpm. This will open a second intake valve for each cylinder so there is a speed but away goes the mpg. Other thing is leave it in gear and coast that way because all the fuel injectors shut down down in that case. Oh and pay attention to your mpg gauge it will show that even a tiny bit of throttle application has a big impact on your mpg. Good luck.