Ken,
Please don't feel insulted if this isn't the right answer. I've missed a
few easy answers myself in my day...

When I have a decent mix of local road and highway I run around 19mpg
but will run 24 or so for long freeway runs and 16 if I never get off
city streets. To see this I reset my mpg gauge at every fill up.
Although I don't know how the @#*! thing is really configured, it seems
that the electronic mpg readout is cumulative since the last reset and
not weighted for 'recent' miles. Is it possible you are looking at your
average mpg over the last several thousand miles? So many other cars
seem to have them running as 'current' mpg which pretty much tells you
only whether you are going uphill or downhill.

If you are actually calculating mileage the old fashioned way off the
odometer and gas pump, then I don't know what to say. FYI, over a long
run (a 1300 mile trip) the electronic indicator showed 24.5 but odometer
and gas station receipts worked out to 23.5.

Here are two back at you:

Does your gauge indicate empty at about 17 gallons down? If so, have you
pushed to see how far beyond 'empty' it goes? I know it says I have a 20
gallon tank, but I also know that such specs are not necessarily
accurate. I'd rather not run until it sputters and say "Oh, that's how
much the tank really holds...".

Even with the temp control full over to cold and no heat on do you get
air through the vent that is noticable warmer than what it really is
outside? My local shop is lost, or at least useless, on that one.

David

1. Lighten up - remove all nonessential heavy load.  E.g. I removed spare
tire, jack, floor mats (replaced with 100% cotton, light), etc.  My unit is
2004 and under roadside assistance for 2years/40K miles and I used only 3K,
i drive little relying more on mass transit  Thats why removing spare was
possible

2. Install aluminum alloy wheels/rims, lightweight.  I did all 4 rims for
$540, amazing.  Most fools pay $1300 from dealer, i bought rims with free
shipping from discounttiredirect.com  for TSW APEX 1565APX404114S76
15X6.5 4-114.3 40HS   SPECIFICALLY FOR 2004 NISSAN SENTRA 1.8/2.5/S SEDAN

3. Drive smart: dont accelerate when approaching intersections and then hard
break like so many young idiots do.  Do not attempt to ram cars in front of
you.

Then you wont need horsepower.  It's not horsepower we need, we need lighter
cars which accelerate faster not onlky due to more fuel being burned, thats
stupide.

Bicycle and subway (train) is my preferenece anyway