Ok over the last few weeks my mpg have gone straight to crap.. I averaged 225 to 3/4 tank. now im getting 140. It has slowly gone down. I did put larger tires on, went from 265/70/17 to 285/70/17. I had these exact same tires on my 97 with 4.6, intake, true duals with no crossover, and af1 and got 210 to 3/4 tank. Im bone stock right now and get 140??? On top of this my idle has been around 900rpms lately, actually right around this whole crappy mpg started. The idle usually was around 650ish. What is causing it to idle so high now? is it possible the truck is running rich now?

 

check your IAC, it may be stuck open... or your butterfly valve on your TB may be stuck open.

 

dam i just cleaned the iac too. i'll check them tomorrow if its not subzero out.