I just had a thought, check your air intake port maybe something is restricting the air flow, leaves, animal nest, road debrie did anything get up in there?? just a possibility !

 

I just started having something similar after getting the "bad cat codes" that I got rid of and have not returned in 3,000 miles. Now my highway mileage is 33 at 65MPH, but 27 at 75MPH, the city MPG has also decreased. I think I'm looking at a bad cat.

 

in the manual it states for the 2.4 "for optimum performance use 91 octane" but if you are not putting the pedal to the floor you won't know the difference

 

Something banging around in my head said that may be the case. That's why I tried to qualify with "I thought".

I have found in a 2.2L that you do get a bit better MPG with 91, but the $/M is not worth it.

 

I did a test on my 2008 2lt with 2.4 and found no difference on all 3 octane levels.

 

not on gas mileage...but on Max HP you would see it

 

I remember my dad's Imperial ran on AV gas 104 Octane, he sometimes "joked" about going over to the Marine airfield to get a free tank "that Phantom won't miss a couple of gallons".

 

Although it's strictly forbidden, and FBO operators can get their tails fined off for doing it, some 100LL Avgas does find its way into muscle and exotic cars.

But your dad's Imperial would have ingested "Jet A" with unhappy results Don.

 

I recently found out that codes may not illuminate your CEL and that EVAP system faults do affect your MPG. Mine seems to have been an intermittent stuck open Purge Vent Solenoid, that is the easy and cheaper one.

The stuck open condition causes the computer to think it's running lean, so it adds more fuel causing an actual rich condition. The computer can only control the fuel not the air in the ideal 14:1 mixture.

This is not only bad for MPG, but also catalytic converters.

 

Actually, I think the Navy had JP8 then, not all Diesel. OOPS, I meant JP5.
One of my jobs is running "fan jet fuel" from the tank farms to the lab for testing. Smells just like gas in the back of the HHR.