2.4 Liter MPG Dropped

Like a rock. I bought the 96 Tacoma with a 2.4 about a year ago. I was pleasantly surprised that I consistently got 23.5 to 25 MPG. This was a tank every 10 days & stayed the same til september. Then, all of a sudden I am consistently getting 20-21 MPG. At first I thought that the cooler weather might be responsible.

After several months of this I did a complete tuneup. Everything I removed was original Toyota.....but the truck only has 51,000 mi. MPG improved, but only to a consistent 21-22. I am thinking maybe O2 sensor???? I'd change them on GP's but at $120.00 EACH I'd like to be certain that they will fix the mileage problem. Any ideas?

Also - is there a check or way to set base timing? I couldn't find anything in the Chiltons I have. Thanx
LASTLY -
What is reasonable MPG to get with the 2.4 4?

I get 28 mpg with mine, I have a 98 with a 5 speed, all stock. I'm assuming that you have an automatic??
I see they sell a diagnostic module at wal mart for 100.00. These things are so computerized now that I think you need to get it hooked up to a computer to see what's going on.
Also, are your plugs burning clean??
I just got mine so I haven[t put my timing light on it yet. Still waiting on the factory service manual on CD. BTW, I've always found Chilton to be a piece of crap, Haynes has consistently had better information, but not as good as the factory manual.