New plugs/wires/coil packs. Now lower mileage?

[Stubby79]

Wrong/heavier grade of oil put in?
As already suggested, alignment?
Perhaps your CAT decided to get plugged up when it wasn't firing right.

[hat_man]

OK. If the battery was disconnected when the mechanic did the plugs and other stuff, does the computer (ECM?) pull a "return to defaults" trick? The last tank was actually better than my 28 mpg that I was used to. Only by a little, but better. This is what I had expected in the first place with new plugs and coil packs. Does the computer "re-learn" some sort of parameters after power is removed? Is this what I hear referred to as "fuel mapping"? Still going to have the alignment checked next week. In for a penny in for a pound.

Thanks for all the suggestions. I'll be back with some updates over the next few weeks.

[Frank Lee]

Fill errors are a real thing but we can't tell your consistency from your fuel log.

[hat_man]

I wish I had more data to give other than my word, but it's all I have. I drive the same route everyday. Very little (if any at all) "errand running". 120-130 miles a day/5 days (depending on the job site) a week for the last 20+ years. 10 years with the Ranger. Every time I fill up it's been within .25/gallon for the tank. A quart of fuel difference in a 12-13 gallon fill shouldn't be cause for too much concern. Before the new rim and tire combination I was doing better, but I have gotten a consistent 28mpg (+/- <1mpg) since the new "shoes".

Again, maybe the truck is having to re-learn something as I have seen improvement since the first abysmal 22mpg until now at a shade better than the 28mpg I was getting before all the work and re-work. I guess only time will tell at this point.