06 3.7 auto 4X4 32" tires
I am getting really poor fuel milage I average 320 kms on a tank 95% city driving. When I do hit the highway it has trouble maintaining speed in overdrive weather I am using cruise or just holding the pedal it drops speed and kicks down constantly.
No MILs or codes present. I have flushed the transmission and changed the filter and replaced plugs (copper), new muffler and air filter.
Ideas?

 

ChRispyX7

If nothing stands out as obvious, have you checked your brakes? Maybe you have a stuck piston or two on the front disk brakes. Just lift the front tires off the ground and try to see if they spin freely by hand.

 

I had a caliper piston fail on me once. Kept the pads from coming off the rotor on the drivers side. Didn't notice a drop in MPG's or anything, but fortunately I got off the highway only few miles into my trip (realizing I needed gas). As I shimmied around the front (b\c I was close to the pump) I almost burned my leg, due to the entire wheel being super HOT!

 

The 3.7 with 32" tires would suffer depending on tearing. I have 3.92 gears with mine and I find it suffers on my 30.5" tires (245/75r16). Do you know what gears you have in your truck?

When I size up next time (265/75/r16 durotrac or KM2), it will include regearing to 4.10 or maybe 4.55.

 

depending on mileage, start truck and put your hand over exhaust tip, see if anything is coming out, if not a partial blocked cat might be the issue.