Mileage after spark plug replacement

[ciaka]

Over the last few months I noticed a drop in mpg, and on top of it, my car just pinged 140K, so decided to do the spark plug change.
Did the change (easy enough) on all 6 plugs.
After the change, I noticed that over a week of driving or so (normal street/hwy mix), the mpg was at between 16-17.
Much worse than what I remember the car should have been for that type of driving (remember it between 18-21mpg of same driving).

Decided to to the air volume relearn, throttle body position relearn, accelerator position relearn. Did that and thought mpg improved, but after monitoring for about a week, the mpg seems to be going down again to about 17mpg or so.

Anyone have ideas on what I can do next to get this rectified?
I thought of going to autozone to hook up a computer to see if there are any codes, and see if they can reset the ecu (can that even be done there?).
Need ideas here for possibilities. The air filter was just replaced too.

Thanks.

[pridbor]

I saw 15.3 the other day and got a little worried that something had gone really bad. Took it on the freeway, reset my display and kept the car at 65 for 5-10 miles and observed around 26 MPG. This is what I normally see going long distances.

My average over the life of the car (280 fillings since new, not 1 missed) is 20.535

My 15MPG reading must have occurred as my sister used the car for running short errands, and she's not accustomed to this large type of cars. She's living back in Europe.

So I suggest you make a test of like nature and see what you achieve. I assume that your trip rendering your low result is some kind of mixed drive, which is always difficult to do consistently

My 2 cents worth

[ciaka]

did that. after reset, mpg goes to about 29mpg (when driving at about 60-70mph). When you add all the traffic street driving, it reduces again to about 17.5mpg. Still think it is much lower than it should be - or am I incorrect in my expectations?

[Eric L.]

I have gotten 17.5 in mixed driving before, quite frequently in fact. That is about what you can expect if you mix in any city driving with freeway trips. I can maybe do 21-22mpg on all freeway. City, maybe 16mpg. Together, 18-19mpg max. But that is just my driving, and I too, overmaintain my car.

[Pilgrim]

In town, 16-17. I love to get away from stoplights and don't mind the gas required to do it. On the freeway between 60 and 80, 21-22 is normal if traffic allows fairly consistent speeds.

[ciaka]

So looks like I am in same ballpark then. I remember it higher for some reason, a few years back. Well, I will keep an eye on it for a while. Thanks for input.

[Pilgrim]

It's always an idea to keep an auto record book and track stuff like this, including occasional gas mileage, oil changes, tire rotations, repairs, etc.

This one is my favorite - pricey but well laid out:

Amazon

Amazon