Can there be codes that the Check Engine light won't come on for? Just grasping at straws. My mileage has dropped from 16.4 to 12.3 city and 23.7 to 17.1 freeway. No hesitation or stumble but that's a huge drop. 5.4 sup crew 2wd. Newish cops plugs, fuel and air filter within the last 6k. Same tire and they are at 36 psi. Cleaned throttle body and maf last week, they were very clean. New oil and trans fluid within 1k, also new belt. I use a vacuum gauge to drive by and the right foot is not the problem. My thought might be O2 sensors, but at 78k, seems early.
Thanks

 

Forgot to mention put on jack stands and all 4 corners spin freely so no caliper drag. Also checked the parking brake and it is not hung up.

 

I hope you don't mind me hanging out here. I'm curious to see what others post.

I have a 06, SCAB, 4.6, 4x4 with a similar issue. I purchased the truck a year ago (60k miles) and averaged 13.8 mpg HWY with 4 snow tires. I changed tires (A/T) and replaced similar items as you have, hoping to see some improvement, but none. I haven't actually calculated my mpg's lately, but based on the miles/tank, I've lost about 50-100 miles/tank.

 

One small possibility is that the gas stations are now pumping summer blends. Not sure if this would lower your MPG by as much as you are experiencing though. Have you switched gas stations? Are you using a gas station that has a higher ethanol mix?

 

24 mpg ??

 

Summer blend usually increases MPG, not decreases it.

 

I could coax almost 24 between 55-65. Above that it drops fast to about 16 mpg at 70 mph.

 

with a 5.4... ummm

 

This is exceptional fuel mileage! On any given highway cruise with my 2007 4.6 V8 with it's 3:55 rear gear (tow package), I see 16.5 MPG at 65 to 70 MPH.