95 2.2 poor mileage and rough starts

I have a 1995 S10, 2wd, 2.2 5 speed with 221,000 miles. I Check the mileage with every fillup and noticed recently that I am only getting 24.5m, where I usually get around 26. I noticed is has had some rough starts recently on cold mornings, but evens out within 5-10 seconds and runs fine.
New plugs, plug wires, air filter, and pcv. O2 senser was replaced once, can't remember when, could easily have been 100,000 miles ago. Recent oil change, been using synthetic for the last 21,000 or so.

My truck has never thrown a code in 221,000 miles, even when I had a bad plug wire and it was missing terribly on one cylinder, so I am getting no help from the computer.

What are the most common causes for poor mileage and rough starts that I haven't already replaced?

Thanks

could be O2 sensor is getting old. do u have access to a scan tool to look at the data? could be a number of things. tires causing more drag for whatever reason. i usually get 17-21mpg in my 98 2.2 5spd. mpg issues can be "fun"/hard to trace. maybe run some injector cleaner and double check fuel pressure and the regulator if equipped. let us know what u find.