Have you heard about SOL for cars?
It's like the Standards of Learning for kids – the tests administered by the schools as a way to gauge whether (cue The Chimp) the children is learning. SOLs are widely considered a scam because the kids aren't learning – just being taught to pass the test.
It's a game.
Likewise, the government's miles-per-gallon testing. The car companies build their cars to perform as well as possible on the EPA's test loop, so they can tout the best-possible city/highway numbers – and not just to entice buyers. These numbers are also used to calculate the Corporate Average Fuel Efficiency (CAFE) numbers which the car companies have to deal with. When their "fleet average" – the combined mileage of all the vehicles they sell – dips below whatever Uncle says the mandatory minimum is (it's currently 35.5 MPG) they get fined and these fines, of course, are passed on to us.
So it's very important to do well on the tests.
This is why almost all new cars come only with automatic transmissions.
These transmissions can be programmed – optimized to perform best – on the EPA's tests. They are set up to shift as soon as possible into overdrive (many new automatics have more than one overdrive gear) which is what you want if you're trying to do as well as possible on the test.
But out in the real world, your mileage may and probably will vary.
An automatic that's programmed to upshift as soon as possible for the sake of maximizing the MPGs does so at the expense of acceleration. The car feels sluggish, flat. This tends to encourage the driver to push harder on the accelerator pedal, in order to force a downshift – in order to get the car to accelerate.
This tends to use more gas.
You might do better with the manual – assuming you know how to drive one. And assuming one's available.
Because of the need to do well on the tests, it's probably not.
Manuals are too variable.