Nissan Altima: Nissan Altima 2.5S

You didn't say, but is it an automatic? How many miles, how long or
miles has it been since it has had new plugs? If its cold(like under
35 degrees or so) it is normal for it to shift a little jerkey until
the transmission fluid gets up to temp. Have it checked or check it
youself to make sure it has the correct level of fluid (DO NOT
overfill it).

Milage will vary, depending on how driven, where driven and what tune
the engine is in. Driving around town, stop and start, idle time and
such your milage may be up to ten miles a gal less than driving at a
steady speed of 70 on the highway using your cruise control.

Keeping the car tuned, using the right weight of oil, the correct psi
in the tires and keeping your foot out of the accelerator your car
will give the best mpg that it can.

Dealers tell lies, look on the net for other peoples gas mileage and
get a general comparison number. But don't feel bad if your a little
under them. Cars are like people, they are all different and no one
person drives like the other or under the same exact road and traffic
conditions.