Mpg - power issues

So I have a 99 4runner 5 spd 3.4:

Originally - bumper, sliders 4.10s 235/85/16 km2

Plenty of power and 18/20 mpg

Changed - 255/85/16 and 4.56s

Mpg - 10/12 and seems gutless


I've also tried 5.29 rear diff and also got the same bad mpg


Things done to fix the problem:
-Changed iac/tps
-Cleaned maf
-Changed front O2 sensor
-Changed plugs to Ngk dual electrode
-no rattle from catalytic converter

Scan gauge shows no codes



Now I'm trying to figure out my next step before I take it to a shop for diagnostic.

Things I'm thinking - maf going bad? Cat going bad? Fpr going bad?

Looks like it's running rich too


Any help or suggestions?

 

You only changed your tires by making them 1" tall and 1" wider so there should be no reason for such a substantial drop in MPG unless...
-What was/is your former and current RPM's? It's possible your gears have taken you out of your powerband.
-Why change gears when you changed tires? 1" isn't substantial unless you were already lugging with the tires you had on there.
-Did you correct the speedo? 1" isn't much but if it was already off, 1" would exaggerate the MPH/MPG readings.

 

It's 2700 rpm at 70 mph vs around 2500.

Speedo is corrected. Changed gears because mtns and the weight of the bumpers/sliders.

 

Thats odd if nothing else changed but tires and gears. Almost makes me wonder if you got some bad gas or plugged your fuel filter.

 

Is this 4x4? If so are you changing both diff gears(front and rear)?

 

Oh yeah and I changed the fuel filter

 

Waiting on the front in the mail. I have manual hubs.

 

as long as hes driving in 2wd it doesnt matter.

thats real weird OP. cant think of anything besides what you have already said.

 

This is true. Wondering if hubs locked in but transfer not engaged.

 

Hubs are free

 

Not a shot at you man.Just seems so drastic.

 

Oh I know, I'm confused myself. I expected at worst 15 mpg.


And super babying it I get 12.5/13

 

Can't you go back to original setup? If it's just tires,you could sell them. Was that the only reason for changing gears? Seems from your post that you had plenty of power and decent mpg. Kinda seems like it's not worth it just for tires. Did you try 4:10 gears with those tires on?

 

I will assume that you checked the air filter.

Ply rating stayed the same on the tires?,,and Mud Terrains again?

You say it looks/smells like it is running rich. Plugs are jet black from over fuel?

 

Air filter is pretty new, both mud terrains, plugs aren't quite jet black.

 

Well talked to my mechanic friend and it seems more and more my MAF might be going out.

Calculated load
2nd gear WOT - peaks at 71 (5000) tapers off at 67 at redline
3rd gear WOT - peaks at 70 (5000) tapes off at 67 at redline

 

Clogged cat? Was there a significant increase in weight with the larger tires?

 

Maybe 5-7 lbs

No cat rattle or codes

 

New plugs are black now,,or the old ones?

How did you clean the MAF?

I will assume that you yanked the positive cable off the batt when doing all this, resetting the system afterwards after all the new and cleaned sensors?.

 

I bought my truck with 33's on it. I don't know what MPG it was getting before, but I too am getting 13 MPG if I'm a lucky guy. lately it's worse but I'm blaming that on the weather haha. If you find that it's the MAF let me know. good thing it's tax season.