Help! 15.5 MPG vs 22 MPG

[ramrod_2023]

Pentastar 3.6 question...I'm consistently getting 15 to 16 MPG in the city.

BUT on the highway, I'm consistently getting 22 MPG, sometimes up to 25!

Is this normal? 126,000 miles on the engine. (Rebuilt transmission installed at about 100,000.) I believe spark plugs and wires replaced about 70,000.

Other interesting piece of info. It ran rough for about a week. I wanted to take it in, but I had to leave town for a 1,000 mile trip. Engine light came on, AS I WAS LEAVING TOWN. About 2 hours into the trip, it went off! The rough idle was virtually gone when I got back home. Meanwhile, I bought a OBDII reader and no error codes. Should I take it in to have it looked at?

Also, is it recommended I should I replace the plugs/wires again? Or is 16 MPH in the city what I should expect?

Thanks for any advice!

 

[SitKneelBend]

Pentastar 3.6 question...I'm consistently getting 15 to 16 MPG in the city.

BUT on the highway, I'm consistently getting 22 MPG, sometimes up to 25!

Is this normal? 126,000 miles on the engine. (Rebuilt transmission installed at about 100,000.) I believe spark plugs and wires replaced about 70,000.

Other interesting piece of info. It ran rough for about a week. I wanted to take it in, but I had to leave town for a 1,000 mile trip. Engine light came on, AS I WAS LEAVING TOWN. About 2 hours into the trip, it went off! The rough idle was virtually gone when I got back home. Meanwhile, I bought a OBDII reader and no error codes. Should I take it in to have it looked at?

Also, is it recommended I should I replace the plugs/wires again? Or is 16 MPH in the city what I should expect?

Thanks for any advice!

Check your parking break cable. If it's sagging, even a little, it could be robbing you of efficiency. The rough idle you experienced happened to me twice, bad fuel injectors. It can clear on its own too once the voltage returns to normal but that may be what you are experiencing. If you take it to a store like AutoZone they may be able to see the stored codes.

 

[Dean2]

Put a can of Seafoam in the gas for the next two fill ups. Does wonders for cleaning the varnish out of injectors. Fixes a LOT of rough idle, misfires and the like, Also, get some throttle body cleaner, do NOT use anything but specific cleaner, and clean out throttle body, MAP sensor etc. Also check or change PCV. All very cheap fixes that make quite a difference on higher mile motors.

With respect to much lower mileage around town, pretty normal. Your is a little more pronounced but if there is lots of stop and go and you like the goose the skinny pedal leaving the lights, then you are right in the normal range. The 2020 Grand Cherokee we had with the 6 showed a ten mpg difference highway to city, and the 2020 5.7 Grand Cherokee we kept actually gets better highway mileage and about the same around town and it shows the same large variation, 21 in town, 30 highway. The pickups don't do as good for mileage as the Cherokee due to weight and poorer aerodynamics.

 

[CamperMike]

That mileage sounds decent. In town is always much worse. My old T&C with the 3.6 got around 16-17 in town and 25 on the highway. Our newer Pacifica gets around the same in town but can get upper 20s on the highway. In-town is just terrible on mileage.

 

[TotallyHucked]

Yeah, that's about right. You have to remember, it's a heavy truck and accelerating all the time uses alot more fuel than steady state cruising.

 

[mb33458]

I average about 19.8 overall, so you are in the ballpark.

 

[Bob1313]

Recent trip to Phoenix driving 65 mph produced 28 mpg.
Next trip driving 75 netted 21.
I have the 5.7

 

[Fightinbass]

It could be the coolant bypass valve aka coolant 3 way valve
When mine went it gave intermittant CEL but no codes
Problem affected mpg