I think my wording was confusing. I did replace the old worn plugs with new oem plugs that were correctly gapped. I was just saying that because the old plugs were so bad I had hoped to see a noticeable increase in mpg when I replaced them but there was no change.
I noticed that most posters here suggest OEM replacement parts. I am just not sure what I should replace first. I don't have the money to just replace everything at once so my plan is just to buy a couple parts each paycheck, replace them, and note whether they made a difference in mpg or not.
I wish there were a list here of common maintenance items that can cause mpg loss or degradation, organized by most likely to least likely, and/or highest possible mpg loss to lowest possible mpg loss.
The reason I recommend OEM is simple. Cheap Auto Zone parts are just that, cheap. The quality of these parts is hit or miss, and a lot of it is made in China crap. Performance parts really don't do much for economy, and are very pricey. The OEM parts have to meet Federal emissions standards, and reasonably outlast the OEM's new vehicle warranty. Having problems with a $50 part costs them a lot more than $50 after the dealer pays their guys to work on the car. Its just cheaper for them to put a quality part in the car.
When I purchase replacement parts, I want the same kind of quality. I want to put a set of plug wires in the vehicle, and not worry about them for the next 100,000 miles, not replacing them again in 10,000 (side note - my worst experience with Chinese parts, and what made me switch to OEM was when I replaced the plug wires in my 74 Chevy pickup. In one week the wires were arcing all over the place, and I had to put the old wires back in just to drive home one night. Luckily the old ones were still in the back of the truck.)
On the OEM issue: I'm usually willing to gamble on cheap parts.
On the issue of what to replace: What's broken, worn out, or overdue for maintenance? Usually I just don't see replacing parts as something that needs to be done for fuel economy unless something has failed.