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