96 Tacoma 2.4L, Gas milage HELP ME!!!!

I have a 96 2.4L Tacoma LX Automatic model. I bought is brand new and it only has 13,000 miles on it. It is still brand new and we never drive. I have recently sold my car and have been driving the truck and the gas miliage has been real bad.
The truck use to be driven about once a week. but now driving it every day it has been getting only 15 to the tank.
15mg and in overdrive only 13.5mg
I do not understand and no lights in the dash have come on.
I dont have a heavy foot either. LOL
What should I do. ? What could it be?
I am having the spark plugs changed tomorrow but am not sure if that is really it.
Ac is never on and I dont punch it. My car had a big v6 engine in it and it got around 22 to the gallon.
the sterring wheel has been viabrating also. What is up with that???
Please if and one has any suggestions. [email protected]
Please tell me what you think it might be thanks, Lompoc, CA 93436

my god, i would try replacing the oxygen sensors
they cost around $20-$25. id say that the steering
wheel vibrating could be the tires. mine needs
replacing on my 99, and my steering wheel
vibrates, i had it checked and the tires had little
knots on the tread both front tires. good luck.