bad mpg after new spark plugs?

Been a long time since I've been on here. Bought a 2010 Tacoma dcsb sport a while back in great shape. Just hit 38k the other day, so I decided to change out the diff fluids, MT oil, transfercase oil and change out the sparkplugs and air filter. When I pulled the plugs, they were def. the originals (ngk 1 side, denso the otherside!) and of course needed to be changed. My mpg wasn't bad before, but now it absolutely sucks. The new plugs I put in were OEM purchased at the local Toyota dealer. I even checked the gaps to make sure they were correct and made sure they all looked ok before installation. I've changed out these plugs quite a few times on these 2nd gen Tacoma's, so it wasn't too difficult. Anyone else experience this? I believe my mpg's have dropped about 2-3 mpg...

 

That doesn't make a whole lot of sense. I can see if you were running the wrong plugs you might see a noticeable difference but you should be seeing better mileage after a tune-up. When you changed the fluids did you put 80W90 in the diffs and t-case or did you use 75W140? Thicker oil causes more drag and would make a difference. Though I doubt it would be 2-3 MPG. Change to heavier tires recently by chance too?

 

So many things can cause it man. Run a couple more tanks of gas through it... every gas station's gas is different too, maybe you got a bad batch? Things just don't sound right, but maybe one gap was off or you dinked it when you installed it?

 

Yeah its def. weird. I re-checked the tightness/torque on all of the plugs about an hour ago and had 2 that we're a little loose. One thing I didn't think about was that I DID fill up at a different gas station than I normally do last week. Maybe I could've got some crappy gas. I re-torqued all the plugs and topped off the fuel tank at the normal place I get gas. We'll see as the wknd progress, maybe it'll get a little better. Oh, and I did use 80-90 gear oil when I changed the fluids out on diffs and transfercase.

 

Subbed to see if it was caused by the fuel

 

That's weird. My MPG didn't change much (if any) when I changed mine at 31k. I gapped my new plugs (bought at the dealer) at .040. My original were worn to .047 passenger side and .045 on the drivers side when I pulled them. I run only Shell V power 93 octane in mine. Like someone else said, maybe you got a bad batch of fuel. Run a few tanks through it and see if it changes.

 

After changed to Denso irdium plugs ($9/plug X 6) for my truck i constantly get 15 mpg combine city/hwy. I am not a lead foot by any mean. Have been thinking about going back to Denso $2 coper plugs.