As the weather cools cars get lower mileage for a number of
reasons. These will vary somewhat from driver to driver and
one car model to another.

Things over which you have no control which will reduce
fuel mileage (in no particular order):
-- Engine computer set to a richer mixer (more fuel) in
winter.
-- Use of a different fuel (per federal law and local
custom) with a lower heating value in winter.
-- Denser air in winter, due to lower temperatures, and so
increasing wind resistance, the #1 robber of fuel efficiency
at highway speeds, summer and winter, etc.
-- Oil viscosity is higher at lower temperatures.

Things you can do that cost nothing and may help improve
fuel mileage:
-- Check tire pressure every two weeks and particularly
during the change of seasons. Also, higher pressures may
result in slightly better mileage. My 91 Civic's tire
pressure is supposed to be 26 psi, but for the past few
years, I have maintained it at 28 psi, for better fuel
efficiency at the expense of a more comfortable ride, which
I don't really notice anyway. It's a Civic, after all.

-- Check the PCV valve. Wrap the hose connecting to it with
cloth, squeeze there with pliers to cut off flow. Listen for
a click, which should happen within 30 seconds. No click,
then spend the bucks and replace the valve. Click, and the
valve may or may not be operating optimally. Consider
cleaning the PCV valve with WD-40 or PB Blaster. The engine
control system relies on a particular metered flow through
this valve for different operating conditions. If
something's clogging it or the spring is old, then it won't
work correctly, blah blah.

-- Check the level of the coolant in the reservoir, per the
owner's manual. Consider doing an air purge of the cooling
system. Certain engine control components are cooled by this
coolant. Wrong levels mean improper cooling mean sub-optimal
operation. Strongly consider a complete coolant drain,
flush, and refill, using only OEM coolant or Havoline Orange
Dexcool.

Things you can do that are not expensive and are probably
worth the money for a car you bought used and don't know
well:
-- Replace fuel filter and air filter
-- Replace distributor cap, rotor, ignition wires, and plugs
with OEM ones. (I don't know if the Bosch ones are the
problem, but they sure do seem suspicious, given your
description.) I wouldn't bother with platinums unless the
owner's manual said to.
-- Wait until the gas tank is near empty and add a bottle of
the fuel injector cleaner "Chevron Techron." Fill tank.

For a car that does mostly highway miles with no other
adverse conditions, I would consider far less frequent oil
changes. Like no more than every 5k miles It's the starts
and stops that are hard on an engine, generally speaking.

Some resources for your new used Honda are listed at:

http://home.earthlink.net/~honda.lioness/id9.html

At a minimum, consider getting an owner's manual, per the
directions at this site, and at least study the maintenance
schedule in it.

One other thing to consider, but not unless you have other
symptoms along with lousy mileage: New O2 sensor. It would
surprise me if a car this young needed one, but it's
something to consider if this problem persists into the warm
seasons.

I don't know about the transmission fluid's effects on
mileage, but if I had the spare money, I think I'd start
fresh with my new used car and replace it. Good investment,
since the transmission is one of the most expensive items on
the car that will mess up if not properly lubricated.

"SK" <skadali@gmail.com> wrote
> Guys
>
> Purchased a used Automatic '97 Honda Civic EX 4 months ago
with just
> 56K miles on it. I drive about 1500 miles/month - 90%
highway miles
> (60-75 mph). I do oil changes promptly at 3k miles. Last
week my car
> hit 60K miles and I changed the Spark Plugs (Firestone
replaced with
> Bosch Platinum plugs). they even said that the wires,
rotor and
> distribution cap are good. Tires and Brakes are good.
Before I changed
> the Spark plugs I used to get 28mpg and now i am getting
only 22mpg. I
> was expecting it to hit atleast 30mpg. I am very concerned
now. Can
> anyone help me understand what the problem is?
>
> One of my friend suggested that I can change the Air
filter - Planning
> to do that soon. Should I replace the transmission fluid,
fuel filter
> or anything else? I use 87octane either at Wawa or Shell.
>
> Thanks in advance for your time
> SK